Electrochemistry of Flotation of Sulphide Minerals

YuehuaHu Wei Sun Dianzuo Wang Electrochemistry of Flotation of Sulphide Minerals YuehuaHu Wei Sun Dianzuo Wang Elec...

Author: Yuehua Hu | Wei Sun | Dianzuo Wang

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YuehuaHu Wei Sun Dianzuo Wang

Electrochemistry of Flotation of Sulphide Minerals

YuehuaHu Wei Sun Dianzuo Wang

Electrochemistry of Flotation of Sulphide Minerals With 287 figures

~ Springer

Authors

Prof. YuehuaHu Schoolof Minerals and Bio-engineering CentralSouthUniversity 410083, Changsha, China E-mail: [email protected]

Prof. Wei Sun Schoolof Minerals and Bio-engineering CentralSouthUniversity 410083, Changsha, China E-mail: [email protected]

Prof. Dianzuo Wang Schoolof Minerals and Bio-engineering CentralSouthUniversity 410083, Changsha, China E-mail: [email protected]

ISBN 978-7-302-18818-6 Tsinghua University Press,Beijing

ISBN 978-3-540-92178-3

e-ISBN978-3-540-92179-0

SpringerDordrecht Heidelberg LondonNew York Libraryof Congress ControlNumber: 2008940827 © Tsinghua University Press,Beijingand Springer-Verlag BerlinHeidelberg 2009 This work is subjectto copyright. All rightsare reserved, whetherthe whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storagein data banks. Duplication of this publication or parts thereof is permittedonly under the provisions of the GermanCopyright Law of September 9, 1965, in its current version, and permission for use must always be obtained fromSpringer. Violations are liableto prosecution underthe GermanCopyright Law. The use of generaldescriptive names,registered names,trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for generaluse.

Coverdesign: Frido Steinen-Broo, EStudio Calamar, Spain Printedon acid-free paper Springeris part of Springer Science+Business Media(www.springer.com)

About Authors

Yuehua Hu Born in 1962, graduated from Central South University (CSU, the former Central South Institute of Mining and Metallurgy) in 1982, got the doctor degree in 1989, and elected as professor in 1991. His professional researches are related with the structure-property of flotation reagents and molecular design, the electrochemistry of flotation of sulphide minerals, the solution chemistry of flotation, the interfacial interaction and fine particle flotation. Hu has acquired the better achievement in above fields and has got many top honors, including 1st or 2nd class National Science & Technology Advancement Award, China Book Award, Chinese Youth Award of Science and Technology, National Scientific Award for Outstanding Youth etc. More than 200 papers had been published in China or foreign countries.Hu was honored as Cheung Kong Scholar of the Ministry of Education, elected as vice-chairman of Mineral Processing Committee of China Nonferrous Metals Society, engaged as the adjunct professor of metallurgical department of University of Utah. Wei Sun Born in 1974, graduated from CSU in 1995, got the doctor degree in 2001. Sun is an associateprofessor at the School of Minerals Processing & Bioengineering of CSu. He had the post-graduate experience at Shanghai Institute of Microsystems and Information Technology, which belongs to Chinese Academy of Science, and at the Department of Materials and Chemical Engineering, Alberta University in Canada. His research fields include: flotation of minerals, flotation chemistry and fine particle flotation or processing. More than 30 papers had been published in China or foreign countries.

Dianzuo Wang Born in 1934, graduated from CSU in 1961, elected as the member of Chinese Academy of Science in 1991, the initial member of Chinese Academy of Engineering in 1994. He became a foreign associate of the National Academy of Engineering (USA) in 1990, the foreign nationality member of Russian Academy of Engineering in 1994, the foreign nationality member of Russian Academy of Science in 2006. He had the position of president of CSU, the position of director of General Chinese Institute of Nonferrous Metal Research, the position of vice-chairman of Chinese Academy of Engineering.

ii

Preface

Sulphide minerals are the important resources of nonferrous metals, which are widely used in many fields such as electronics, chemical engineering, aviation, transportation, construction and metallurgy etc. Flotation technology is the most important method to beneficiate sulphide ores to obtain concentrate containing nonferrous metals since it has been used in industry at the end of the 19th century. As early as in the 1930s-1950s, considerable amount of fundamental research had been conducted on the flotation of sulphide minerals based on wettability, electrokinetics and chemical interactions. The "chemical reaction" hypothesis based on the solubility product of metal-collector salts involved and competitive adsorption based on the interaction of thio-collectors and hydroxide ions with sulphides were proposed to explain the flotation mechanism of sulphide minerals. A mixed potential model has been established, in which there existed a potential at which an anodic reaction of xanthate to form metal xanthate or dithiolate and the cathodic reduction of oxygen proceed at finite rates, underlying the basis of the researches on the interaction mechanism between thio collectors and sulphide minerals in 1950s. Since then, it was realized that the electrochemistry was very important in flotation of sulphide minerals as it determined the oxidation and dissolution of mineral particles in the pulp, the electrochemical equilibria of reagents, electrochemical interactions among reagents with both soluble and surface species of minerals and the galvanic interactions among grinding media, mineral and reagents. The efficiency of flotation and separation of sulphide minerals and consumption of reagents are thus controlled by the electrochemistry of the pulp. The potential controlled flotation technology has been developed and used in industry recently to obtain higher efficiency of flotation separation in many copper, lead, zinc sulphide mines in China. In this book, a general review of the fundamental research on flotation electrochemistry of sulphide minerals is made first in Chapter 1. Chapter 2 to Chapter 9 mainly summarize the results of basic research in our group focused on the topics of collectorless floatability of sulphide minerals and hydrophobic iii

entity, interaction mechanisms between thio-collectors and sulphide minerals, role of oxygen in flotation of sulphide minerals and galvanic interactions in potential controlled flotation systems etc. The various electrochemical measurements, electrochemical corrosive method, electrochemical equilibrium calculations, surface analysis, semiconductor energy band and molecular orbital theory, have been used in our studies and introduced in this book. The collectorless and collector-induced flotation behavior of sulphide minerals and mechanism in various flotation systems have been discussed. The electrochemical corrosive mechanism, mechano-electrochemistrical behavior and the molecular orbital approach to flotation of sulphide minerals will provide much new information to the researchers in this area. The examples of potential controlled flotation separation of sulphide ores, especially the examples of industrial application, listed in Chapter 10 demonstrate the future promise of flotation electrochemistry of sulphide minerals for industrial applications. The authors would like to acknowledge the help of many students and associates in the mineral processing: Dr. Runqing Liu who helped in editing the book, Dr. Guohua Gu, Dr. Shuiyu Sun, Dr. Qin Zhang, Dr. Yunlan Yu and Dr. Daoling Xiong who organized materials for some chapters.

Professor Yuehua Hu Dr. Wei Sun Professor Dianzuo Wang Central South University Changsha, 410083, China

iv

Contents

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals 1.1 Three Periods of Flotation of Sulphide Minerals 1.2 Natural Floatability and Collectorless Flotation of Sulphide Minerals 1.3 Role of Oxygen and Oxidation of Sulphide Minerals in Flotation 1.4 Interactions between Collector and Sulphide Minerals and Mixed Potential Model 1.5 Effect of Semiconductor Property of Sulphide Mineral on Its Electrochemical Behavior 1.6 Electrochemical Behaviors in Grinding System 1.7 The Purpose of This Book

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals 2.1 2.2

Crystal Structure and Natural Floatability Collectorless Flotation 2.2.1 Effect of Pulp Potential on Flotation at Certain pH 2.2.2 Pulp Potential and pH Dependence of Collectorless Floatability 2.3 Electrochemical Equilibriums of the Surface Oxidation and Flotation of Sulphide Minerals 2.3.1 The Surface Oxidation of Sulphide Minerals and Nemst Equation 2.3.2 Electrochemical Equilibriums in Collectorless Flotation 2.3.3 Eh-pH Diagrams of Potential and pH Dependence of Flotation 2.4 Electrochemical Determination of Surface Oxidation Products of Sulphide Minerals 2.5 Surface Analysis of Oxidation of Sulphide Minerals

Chapter 3 Collectorless Flotation in the Presence of Sodium Sulphide 3.1

Description of Behavior

1 1 3 7 8 12 14 19

20 20 23 23 24 28 28 30 . 32 41 48

53 53

v

3.2 Nature of Hydrophobic Entity 3.3 SurfaceAnalysis and Sulphur-Extract 3.4 Comparisonbetween Self-Inducedand Sodium Sulphide-Induced CollectorlessFlotation Chapter 4 Collector Flotation of Sulphide Minerals 4.1 Pulp Potential Dependence of Collector Flotation and HydrophobicEntity 4.1.1 Copper Sulphide Minerals 4.1.2 Lead Sulphide Minerals 4.1.3 Zinc Sulphide Minerals 4.1.4 Iron Sulphide Minerals 4.2 Eh-pH Diagrams for the Collector/Water/Mineral System 4.2.1 Butyl Xanthate/Water System 4.2.2 Chalcocite-Oxygen-Xanthate System 4.3 SurfaceAnalysis 4.3.1 UV Analysis of CollectorAdsorption on Sulphide Minerals 4.3.2 FTIR Analysis of Adsorption of Thio-Collectors on Sulphide Minerals 4.3.3 XPS Analysis of CollectorAdsorption on Sulphide Minerals

57 60 62 63 65 65 69 82 86 91 92 94 95 96 99 109

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals 112 5.1 Electrochemical Depressionby Hydroxyl Ion 112 5.1.1 Depression of Galena and Pyrite 113 5.1.2 Depression of Jamesonite and Pyrrhotite 117 5.1.3 Interfacial Structure of Mineral/Solutionin Different pH Modifier Solution 118 5.2 Depressionby Hydrosulphide Ion 122 5.3 Electrochemical Depressionby Cyanide 123 5.4 Depression by Hydrogen Peroxide 124 5.5 Depression of Marmatite and Pyrrhotite by Thio-Organic Depressants 125 5.6 Role of Polyhydroxyl and Poly Carboxylic Xanthate in the Flotation of Zinc-Iron Sulphide 129 5.6.1 Flotation Behavior of Zinc-Iron Sulphide with Polyhydroxyl and Polycarboxylic Xanthate as Depressants 129 '5.6.2 Effect of Pulp Potential on the Flotation of Zinc-Iron Sulphide in the Presence of the Depressant 131 5.6.3 Adsorption of Polyhydroxyl and PolycarboxylicXanthate on Zinc-Iron Sulphide 133 5.6.4 Effect of Polyhydroxyl and Polycarboxylic Xanthate on the Zeta Potential of Zinc-Iron SulphideMinerals 136 vi

5.6.5 Structure-Property Relation of Polyhydroxyl and Polycarboxylic Xanthate Chapter 6 Electrochemistry of Activation Flotation of Sulphide Minerals 6.1 Electrochemical Mechanism of CopperActivating Sphalerite 6.2 Electrochemical Mechanism of CopperActivating Zinc-Iron Sulphide Minerals 6.2.1 Activation Flotation 6.2.2 Effect of Pulp Potentialon ActivationFlotationof Zinc-Iron Sulphide Minerals 6.2.3 Electrochemical Mechanism of CopperActivating Marm.atite 6.2.4 SurfaceAnalysis of Mechanism of CopperActivating Marm.atite 6.3 Activationof Copper Ion on Flotation of Zinc-Iron Sulphide Mineralsin the Presenceof Depressants 6.3.1 Effect of Depressant on the CUS04 Activating Flotation of Zinc-Iron Sulphide Minerals 6.3.2 Influenceof Pulp Potential on the Copper Ion Activating Flotationof Zinc-Iron Sulphide Minerals in the Presence of Depressant 6.3.3 Zeta Potential of Zinc-Iron Sulphide Minerals in the Presenceof FlotationReagents 6.4 SurfaceChemistryof Activation of Lime-Depressed Pyrite 6.4.1 Activation Flotationof Lime-Depressed Pyrite 6.4.2 SolutionChemistry Studies on ActivationFlotation of Lime-Depressed Pyrite 6.4.3 The Bondingof the ActivatorPolar Group with SurfaceCation 6.4.4 SurfaceAnalysis of Lime-Depressed Pyrite in the Presenceof Activator Chapter 7 Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals 7.1 Corrosive Electrochemistry 7.1.1 Conceptand Significance of Mixed Potential, Corrosive Potentialand StaticPotential 7.1.2 The Conceptof Corrosive Currentand Corrosive Speed 7.1.3 The Corrosion Inhibitor, InhibitingCorrosive Efficiency and Its Relationship with CollectorAction 7.2 Self-Corrosion of SulphideMinerals

137

142 142 146 146 147 149 150 152 152

155 157 159 159 161 163 165

167 167 168 169 170 170 vii

7.3 Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under Different Conditions 7.3.1 The Oxidation of Pyrite in NaOH Medium 7.3.2 Oxidation of Pyrite in Lime Medium 7.3.3 Corrosive Electrochemistry Study on Interactions between Collector and Pyrite 7.3.4 Interaction between Collector and Pyrite in High Alkaline Media 7.4 Corrosive Electrochemistry on Surface Redox Reaction of Galena under Different Conditions 7.4.1 The Oxidation of Galena in NaOH Solution 7.4.2 The Effect of Lime on the Oxidation of Galena 7.4.3 Corrosive Electrochemistry Study on Interactions between Collector and Galena 7.4.4 Interactions between Collector and Galena at High pH 7.5 Corrosive Electrochemistry on Surface Redox Reaction of Sphalerite in Different Media 7.5.1 Influence of Different pH Media on Sphalerite Oxidation 7.5.2 Inhibiting Corrosive Mechanism of Collector on Sphalerite Electrode Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals 8.1 Experiment Equipment. 8.2 Mechano-Electrochemical Behavior of Pyrite in Different Grinding Media 8.3 Mechano-Electrochemistry Process of Galena in Different Grinding Media 8.4 Influence of Mechanical Force on the Electrode Process between Xanthate and Sulphide Minerals 8.5 Surface Change of Sulphide Minerals under Mechanical Force 8.5.1 Surface Change of the Pyrite under Mechanical Force 8.5.2 Surface Change of Sphalerite in Mechanical Force Chapter 9 Molecular Orbital and Energy Band Theory Approach of Electrochemical Flotation of Sulphide Minerals 9.1 Qualitative Molecular Orbital and Band Models 9.2 Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface 9.2.1 Computation Methods 9.2.2 Bulk FeS2 Properties 9.2.3 Property ofFeS2 (100) Surface 9.2.4 Oxygen Adsorption viii

172 172 175 178 183 186 186 187 190 195 197 197 198

201 202 203 208 213 215 215 217

219 219 220 221 223 225 227

9.3 Density Functional Theory Research on Activation of Sphalerite 228 9.3.1 Computational Methods 229 9.3.2 Bulk ZnS Properties 230 232 9.3.3 Relaxation and Properties ofZnS (110) Surface 9.3.4 Relaxation and Properties ofZnS (110) Surface Doped with Cu2+ and Fe2+ ...•..•••••. :•.•......••••.••.••....•••.•••••••••.... 234 9.3.5 Effects of Doped Ions on Mixed Potential / 237 9.4 The Molecular Orbital and Energy Band Discussion of Electrochemical Flotation Mechanism of Sulphide Minerals 238 9.4.1 Frontier Orbital of Collector and Oxygen 238 9.4.2 The Molecular Orbit and Energy Band Discussion of Collectorless Flotation of Galena and Pyrite 240 9.4.3 The Molecular Orbit and Energy Band Discussion of Collector Flotation of Galena and Pyrite 241 Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals 10.1 Technological Factors Affecting Potential Controlled Flotation Separation of Sulphide Ores 10.1.1 Potential Modifiers 10.1.2 pH Modifier 10.1.3 Frother 10.1.4 Conditioning Time 10.1.5 Surface Pretreatment 10.1.6 Grinding Environment 10.2 Flotation Separation of Sulphide Minerals and Ores 10.2.1 Copper Sulphide Minerals and Ores 10.2.2 Lead-Zinc-Iron-Sulphide Minerals and Ores 10.3 Applications of Potential Control Flotation in Industrial Practice 10.3.1 Original Potential in Grinding Process 10.3.2 Effect of Lime Dosage on "Original Potential" 10.3.3 Coupling with Other Flotation Process Factors 10.3.4 Coupling with Reagent Schemes 10.3.5 Coupling with Flotation Circuit 10.3.6 Applications ofOPCF Technology in Several Flotation Concentrators

244 244 244 246 248 249 250 250 253 253 257 258 258 259 260 261 262 262

References

269

Index of Terms

286

Index of Scholars

295 ix

Chapter 1

General Review of Electrochemistry of Flotation of Sulphide Minerals

Abstract This chapter reviews the development of froth flotation achieved in the past one hundred years and accounts for the achievements of the theory of flotation of sulphide minerals in four aspects, which are the natural flotability of sulphide minerals, the role of oxygen in the flotation of sulphide minerals, the interaction of collectors with sulphide minerals, the effect of the semi-conductor property of sulphide minerals and electrochemical behaviors in the grinding system. Furthermore, the purpose of this book is revealed in the end. Keywords

sulphide minerals; flotation; electrochemistry

1.1 Three Periods of Flotation of Sulphide Minerals The one hundred year history of froth flotation may be classified into three periods. The earliest stage is from the end of the 19th century to the early zo" century, i.e. surface flotation or bulk oil flotation. The natural hydrophobic sulphide minerals can be collected by the addition of oil. Froth flotation came into practice in 1909 with the use of pine oil, mechanical flotation machine in 1912, and xanthate and aerofloat as collectors in 1924-1925 (Gaudin, 1932; Sutherland and Wark, 1955). In the 2nd period ranging from the 1930s to the 1950s, basic research on flotation was conducted widely in order to understand the principles of the flotation process. Taggart and co-workers (1930, 1945) proposed a "chemical reaction" hypothesis, based on which the flotation of sulphide minerals was explained by the solubility product of the metal-collector salts involved. It was plausible at that time that the floatability of copper, lead, and zinc sulphide minerals using xanthate as a collector decreased in the order of increase of the solubility product of their metal xanthate (Karkovsky, 1957). Sutherland and Wark (1955) paid attention to the fact that this model was not always consistent with the established values of the solubility products of the species involved. They believed that the interaction of thio-collectors with sulphides should be considered as adsorption and proposed a mechanism of competitive adsorption between xanthate and hydroxide ions, which explained the Barsky empirical relationship between the upper pH limit of flotation and collector concentration. Gaudin (1957) concurred with Wark's explanation of this phenomenon. Du Rietz

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

(1957) considered the formation of metal xanthate as the adsorption mechanism of thio-collectors on sulphide minerals and used the Bjerrum diagram to explain the floatability and the upper pH limit of flotation of sulphide minerals. The adsorption of undissociated acid form of the collector was ever noted by Wark and Cox (1933), Cook and Nixon (1950), and Cook and Wadsworth (1957). In this period, the collectorless floatability of sulphide minerals was referred to as the natural floatability and thought to be typical of sulphides by some investigators (Ravitz and Porter, 1933). Some authors demonstrated that sulphide minerals were not naturally floatable and the so-called natural floatability might have originated from contamination (Hayes et aI., 1987). At the same time, the importance of the presence of oxygen for sulphide minerals to float with xanthate collectors was first revealed by Salamy and Nixon (1953, 1954). They proposed that the interaction between the collector and oxygen at the mineral surface takes place by separate electrochemical reactions which proceed simultaneously. An anodic oxidation reaction involving the collector transfers electrons to the mineral and these are returned to the solution phase by the cathodic reduction of oxygen. This model was supported by investigations that showed the presence of oxygen to be necessary for flotation of sulphide minerals with xanthate (Plaksin and Bessonov, 1957). Thus, Nixon (1957) recognized that the theories of flotation could be reconciled by the electrochemical approach. Since the 1960s', various electrochemical methods such as linear potential sweep voltammetry, cyclic voltammetry etc. and various surface analysis apparatuses such as infrared spectra, X-ray photoelectron spectroscopy etc. have been developed to investigate the electrochemical reaction mechanism involved in the flotation of sulphide minerals (Fuerstenau et aI., 1968; Woods, 1976; Ahmed, 1978; Sun, 1990; Feng, 1989; Buckley, 1995; Arce and Gonzalez, 2002; Bulut and Atak, 2002; Costa et aI., 2002). Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et aI. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et aI., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984; Heyes and Trahar, 1984; Hayes et aI., 1987; Wang et aI., 1991b, c; Hu et aI., 2000). 2

1.2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

In the aspects of the interaction between thio-collectors and sulphide minerals, a mixed potential model was established, in which there existed a potential at which an anodic reaction of xanthate to form metal xanthate or dithiolate and the cathodic reduction of oxygen proceed at finite rates, underlying the basis of the

researches on the interaction mechanism between thio collectors and sulphide minerals (Fuerstenau et aI., 1968; Woods, 1971; Rubio and Kitchener, 1964; Allison et aI., 1972; Richardson and Maust, 1976). "Potential control flotation theory", either in collector flotation or in collectorless flotation, has been gradually established and accepted, in which the flotation of sulphide minerals is possible only under certain redox potential and the separation of poly-metal sulphide minerals can be accomplished by Eh control. The Eh control flotation of sulphide minerals are investigated based on the knowledge including flotation tests, electrochemical measurements, electrochemical equilibrium calculations, various surface analysis, molecular orbital theory, semiconductor energy band theory, crystal chemistry and has been found to be applicable to mineral industry (Hayes and Ralston, 1988; Wang et al., 1992a; Gu et al., 2002a,c, 2004).

1.2 Natural Floatability and Collectorless Flotation of

Sulphide Minerals

The inherent hydrophobicity once thought to be typical of sulphides (Ravitz and Porter, 1933) is now thought to be restricted to sulphides such as molybdenite (Chander et aI., 1975) and other minerals or compound with special structural feature (Gaudin et aI., 1957b). Common commercial sulphide minerals, which are needed to recover in flotation, are normally composed of anion (S2-) and heavy metal ions such as c.', Cu2+, Pb2+, Zn2+, Hg+, Sb3+, Bi3+; transitive metal ion such as Fe2+, C02+, Ni2+; and noble and rare metal ions such as Ag+, Au+, 4+. M0 On the basis of "structural pattern" or mode of linkage of the atoms or polyhedral units in space, Povarennyk (1972) introduced a crystallochemical classification of sulphide minerals, which have six major patterns as shown in Table 1.1. The most controversial and contradicting problem is, perhaps, the natural and collectorless floatability of sulphide minerals. Gaudin (1957) classified the natural hydrophobicity of different minerals according to their crystal structure and showed that most sulphide minerals were naturally hydrophobic to some extent, which had been further proved based on van der Waals theory by Chander (1988, 1999). Lepetic (1974) revealed the natural floatability of chalcopyrite in dry grinding. Finklestein (1975, 1977) demonstrated that orpiment, realgar and molybdenite were naturally floatable, and that pyrite and chalcopyrite had natural floatability at certain conditions due to the formation of surface elemental sulphur. Buckley and Woods (1990, 1996) attributed the natural floatability of chalcopyrite 3

Chapter 1 General Review of Electrochemistry of Flotation of SulphideMinerals Table 1.1

Thesulphide classification scheme (Vanghan andCraig, 1978; Povarennyk, 1972)

Subclass Coordination

Division Simple e.g. Troilite (FeS) group Galena (PbS) group Complex e.g. Polydymite ((Co, Ni)(Co, Ni)2S4) group Pentlandite((FeNi)9Sg) group Bornite (CuSFeS4) group

Framework

Insular Ring Chain

Simple e.g. Argentite (Ag2S) group Complex e.g. Tetrahedrite (Cu12Sb4S13) group e.g. Pyrite-marcasite (FeS2) group Cobaltite-arsenopyrite (CoAsS-FeAsS) group e.g. Realgar (AS4 S4) group Simple e.g. Stibnite(Sb2S 3) group Millerite (NiS) group Complex e.g. Berthierite(FeSb2S4) group Lautite (CuAsS)group

Layer

Simple e.g. Molybdenite(MoS2) group Complex e.g. Covellite (CU2CUS2S) group

to the formation of metal-deficient poly sulphide, which was further tested by other reports (Chander et aI., 1988; Woods and Yoon, 1994; Woods et aI., 1994). Most sulphide minerals do not exhibit natural floatability. They have, however, self-induced collectorless floatability in specific pulp potential ranges, under the condition of which the sulphide mineral surface has been rendered hydrophobic by the surface self-oxidation. The recovery- size curves of 8 common sulphide minerals in the absence of conventional collectors but at favorable levels of pulp potential (Ept) and pH ranked collectorless floatability of some sulphides, in descending order are as: chalcopyrite> galena> pyrrhotite> pendlite > covellite > bornite> sphalerite> pyrite> arsenopyrite. The floatability of the first four minerals is strong over a wide size range. Whereas, the floatability of the last four minerals is weak in descending order from bornite to inactivated sphalerite. Covellite floats somewhat like bornite and arsenopyrite is similar to pyrite (Trahar et aI., 1983; Guy and Trahar, 1985). Many authors indicated that self-induced flotation of sulphide minerals can occur only under moderately oxidizing environments. It is obvious that the 4

1.2 Natural Floatability and Collectorless Flotationof SulphideMinerals

self-induced collectorless flotation of many sulphide minerals is controlled by pulp potential (Heyes and Trahar, 1977; Gardner and Woods, 1979; Hayes et al., 1987; Hayes and Ralston, 1988; Cheng and Iwasaki, 1992; Sun, 1990; Wang et al., 1991, 1992; Yekeler and Sonmez, 1997; Ekmekci and Demirel, 1997; Zhang

Qin et al., 2004c). The influence of pulp potential on the floatability of galena, arsenopyrite, pyrrhotite and pyrite was reported and given in Fig. 1.1 and Fig. 1.2. Galena, arsenopyrite, and pyrrhotite show strong self-induced collectorless flotation in certain range of potential although these minerals also exhibit different upper and lower potential limit for flotation. In Fig. 1.2, pyrite shows no sign of self-induced flotation, while other investigator claimed self-induced flotation of pyrite, especially in strong acidic pH range (Heyes and Trahar, 1984; Sun et al., 1992,1993). 100 80 ~

~

'6

60

---Galena -...... Arsenopyrite pH=6 PEO=lO mg/L

Q)

>

0

o Q)

et:

40 20

o

0.2

0.4 0.6 0.8 Potential EPt (V, SHE)

1.0

Figure 1.1 Effectof pulp potential on self-induced collectorless flotation behaviors of galena and arsenopyrite at pH = 6 (Sun, 1990) 120 Pyrrhotite

100 ~

~

80

~ ;>

60

~

40

o

~

20

o

pH=8 PEO=400 mg/L Ceramic mill

.

• •

...

Pyrite

.

O'----..L.._--L-_L....--.L.._......L..----I

-0.4 -0.2 0 0.2 0.4 0.6 Potential EPt(V, SHE)

0.8

Figure 1.2 Effectof pulp potential on self-induced collectorless flotation behaviors of pyrrhotiteand pyrite (Heyes and Trahar, 1984)

It has been reported that although pyrite is only slightly floatable in self-induced flotation, the floatability is pronounced if sodium sulphur is added, which is called sulphur-induced collectorless flotation. As can be seen from 5

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

Fig. 1.3, sulphur-induced flotation of pyrite was observed at pH = 8 if the concentration of the sulphur during conditioning was 10-3 mol/L or greater, but there was no significant flotation when the concentration of the sulphur was below this level (Trahar, 1984). Reyes and Trahar (1984) leached pyrite with cyclohexane and compared the extract with a sulphur-containing solution of cyclohexane in a UV spectra photometer as shown in Fig. 1.4, indicating that sulphur was present at the mineral surface. Therefore, the inherent hydrophobicity and natural floatability once thought to be typical of sulphides is now thought to be restricted to sulphides such as molybdenite and other minerals or compound with special structural features. The collectorless floatability that most sulphide minerals showed came from the self-induced or sulphur-induced flotation at certain pulp potential range and certain conditions. 100

.-----,.,"

80

//" - - - - Woodlawn Py 10-3 mol/(L. S2) I I - - Renison Py " 10-3 mol/(L. S2) RenisonPy 40 I /

!

.

I

20

.

~

o

2

468

10

Time (min)

Figure 1.3 Recovery-time data for sulphur-induced flotation of pyrite at pH = 8 (Trahar, 1984)

1.0 0.8 Q)

~ 0.6 '€o

Cyclohexane extract of pyrite

~ 0.4 0.2 O'--_...L--_~_--L-_---'_----'

220

240

260

280

300

320

Wave length(nm)

Figure 1.4 Comparison of UV spectrum of cyclohexane extract of sulphidized pyrite after flotation with that of cyclohexane solution of sulphur (Reyes and Trahar, 1984)

6

1.3 Role of Oxygen and Oxidation of Sulphide Minerals in Flotation

1.3

Role of Oxygen and Oxidation of Sulphide Minerals in Flotation

In the early stage of flotation, the presence of oxygen was considered to be harmful to flotation. Gaudin (1957) considered that the oxidation of sulphide surface produced oxygen-sulphur species, which exchanged further with the collector ion to form metal collector salt. Plaksin et al. (1957, 1963) found that the presence of oxygen consumed the free 'electrons on the galena surface and changed the galena surface from n-type to p-type, which resulted in the adsorption of xanthate ion due to the presence of hole onp-type galena semiconductor. Most importantly, Tolun and Kitchener (1963, 1964), Fuerstenau et al. (1968) and Majima (1968, 1969) found that the presence of oxygen as a cathodic component was indispensable for the electrochemical reaction on sulphide surface based on electrochemical theory. These works underlay the basis of the later "mixed potential model". In 1975, Biegler et al. (1975) investigated the reduction of oxygen on the surface of different types of pyrite using rotating disc electrode technique and found that the difference of reduction behavior of oxygen was due to the different surface electronic structure. Rand (1975) reported the reduction behavior of oxygen on chalcopyrite, galena, arsenopyrite, covellite, chalcocite, pyrite etc. to show that oxygen was most active at the surface of pyrite and inert at the surface of galena. He considered that the difference of cathodic reduction of oxygen affected the adsorption of collector on mineral surfaces, which was further proved by Neeraj (2000) in the light of their researches using electrochemical impedance spectroscopy. Ahlberg and Broo (1988, 1996, and 1997) studied the oxygen reduction on gold, platinum, pyrite and galena in the presence of xanthate using rotating disc electrode technique. They found that the product of oxygen reduction was dominant as perhydroxide on pyrite surface and as water on galena surface. The current of the cathodic process of oxygen reduction was greater on pyrite than on galena and the formation of dixanthogen on pyrite may relate to the presence of hydrogen peroxide. Recently, more researches focused on the oxidation products and the rate of sulphide mineral surface. It was reported that the oxidation products of sulphide mineral were affected by pulp potential and pH as well as time. At relative low potential, the surface of galena was oxidized to produce elemental sulphur resulting in collectorless flotation. At high potential, the surface of galena was oxidized to produce lead sulphate at pH = 6, lead thiosulphate at pH = 9 and lead hydroxide and sulphate at pH>12. Pyrite was oxidized to form ferrous hydroxide and sulphate at the initial stage and further to ferric hydroxide in pH = 9.2 buffer solution. It was generally agreed that the oxidation rate of sulphide minerals were faster at high potential than at low potential. The oxidation rate of sulphide minerals were in the order of pyrrhotite> pyrite> chalcopyrite> sphalerite> galena (Feng, 1989; Qin, 1998; Kelsall and Yin, 1999). Brion (1980) reported that the oxidation rate of chalcopyrite and pyrite greater than that of galena and sphalerite in terms 7

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

of the results ofXPS and polarized curve. Therefore, it has been concluded that the reduction of oxygen as a cathodic process was essential for the electrochemical reaction on sulphide surface and was different for various sulphide minerals. The reduction of oxygen affected the oxidation of sulphide minerals and the interactions with collectors, which had a pronounced influence on flotation behavior of sulphide minerals (Ahmed, 1978; Buckley et al., 1985, 1995; Woods, 1984, 1994; Hu et al., 2004; Yu et al., 2004a; Zhang et al., 2004a, d).

1.4

Interactions between Collector and Sulphide Minerals and Mixed Potential Model

Before the establishment of the "mixed potential mode", there was much controversy on the interactions between thio-collector and sulphide minerals. The chemical reaction, adsorption of ion or undissociated acid had ever been suggested. Since the 1960s' the mode that an anodic oxidation reaction involving the collector transfers electrons to the mineral and these are returned to the solution phase by the cathodic reduction of oxygen was supported by many investigations (Fuerstenau et al., 1968; Woods, 1971, 1976; Allison, 1972; Goold, 1972; Richardson, 1976; Trahar, 1984; Hu et al., 2000; Yu et al., 2004b,c). According to this model, the electrochemical reactions between thio-collector and sulphide mineral included the cathodic reduction of oxygen in Eq. (1-1) (1-1) and the anodic process that the collector transfers electron to the mineral in different ways shown in Eqs. (1-2), (1-3) and (1-4). The electrochemical adsorption of thio-collector ion X- ~Xads+e-

(1-2)

The oxidation of the thio-collector to its dithiolate (1-3) The chemisorption reaction mechanism MS+2X- ~MX2+S+2e-

(1-4a)

MS + 2X- + 4H 20 ~ MX 2+ S04 2-+ 8H++ 8e-

(1-4b)

2MS + 4X- + 3H20 ~ 2MX 2+ S2032- + 6H++ Se

(1-4c)

where X- represents a thio ion and MS represents a sulphide mineral. 8

1.4 Interactions between Collector and Sulphide Minerals and Mixed Potential Model

The overall reactions were (1-5) or -

MS +2X +

1 2

-02~MX2+S+H20

MS + 2:A + 202~ MX2+ S0422MS + 4X- +

~02 ~ 2MX 2+ s2ol2

(1-6a) (1-6b) (1-6c)

For overall reactions such as (1-5) or (1-6) to proceed, there must be a potential, termed a mixed potential, at which the anodic reactions, such as (1-3) and (1-4) and the cathodic reaction of oxygen proceed at finite rates. The mixed-potential model demonstrated the importance of electrode potential in flotation systems. The mixed potential or rest potential of an electrode provides information to determine the identity of the reactions that take place at the mineral surface and the rates of these processes. One approach is to compare the measured rest potential with equilibrium potential for various processes derived from thermodynamic data. Allison et al. (1971, 1972) considered that a necessary condition for the electrochemical formation of dithiolate at the mineral surface is that the measured mixed potential arising from the reduction of oxygen and the oxidation of this collector at the surface must be anodic to the equilibrium potential for the thio ion/dithiolate couple. They correlated the rest potential of a range of sulphide minerals in different thio-collector solutions with the products extracted from the surface as shown in Table 1.2 and 1.3. It can be seen from these Tables that only those minerals exhibiting rest potential in excess of the thio ion/disulphide couple formed dithiolate as a major reaction product. Those minerals which had a rest potential below this value formed the metal collector compounds, except covellite on which dixanthogen was formed even though the measured rest potential was below the reversible potential. Allison et al. (1972) attributed the behavior to the decomposition of cupric xanthate. The electrochemical studies of pyrite/oxygen/xanthate have been carried out using polarographic method and potential sweep technique by many researchers (Majima and Takeda, 1968; Usul and Tolun, 1974; Janetski et al., 1977; .Gardner and Woods, 1977). They found that an anodic current which commenced at a potential close to the reversible value for the xanthate/dixanthogen couple. The typical results investigated by Gardner and Woods (1977) are shown in Fig. 1.5. It shows that the oxidation of each xanthate begins at a potential close to the equilibrium potential of the corresponding xanthate/dixanthogen couple, and that the finite contact angles are formed only at potential where dixanthogen is formed: They also found that a monolayer of dixanthogen was required to float a pyrite particulate bed with ethyl xanthate at pH = 9.2. 9

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals Table 1.2 Products of the interaction of sulphide minerals with potassium ethyl xanthate and the measured rest potential. Potassium ethyl xanthate (6.25 x 10- 4 mol/L at pH = 7) reversible potential for oxidation to dixanthogen is 0.13 V (Allison et al., 1972) Rest potential (V)

Products

pyrite

Minerals

0.22

dixanthogen

arsenopyrite

0.22

dixanthogen

pyrrhotite

0.21

dixanthogen

molybdenite

0.16

dixanthogen

chalcopyrite

0.14

dixanthogen

alabandite

0.15

dixanthogen

covellite

0.05

dixanthogen

bornite

0.06

metal xanthate

galena

0.06

metal xanthate

Table 1.3 Products of the interaction of sulphide minerals with sodium dimethyl dithio carbamate (5.8 x 10- 4 mol/L at pH = 8) and the measured rest potential (reversible potential for oxidation to thiouram disulphide is 0.176 V) (Finkelstein and Goold, 1972) Minerals pyrite

Rest potential (V)

Products

0.475

disulphide

covellite

0.115

metal dithiocarbamate

chalcopyrite

0.095

metal dithiocarbamate

galena

-0.035

metal dithiocarbamate

bornite

-0.045

metal dithiocarbamate

chalcocite

-0.155

metal dithiocarbamate

Guy and Trahar (1984) compared the flotation data with the voltammetric results given by Woods (1971), as shown in Fig. 1.6. It can be seen from Fig. 1.6 that if galena is initially reduced, flotation of galena began in the same potential region as an anodic peak occurred on the voltammograms. If galena was ground in an oxidizing environment, the anodic current appeared at about -0.5 V at which flotation occurred, which was attributed to the formation of lead xanthate from lead in the surface formed by the oxidation conditions. Some instrumental methods have been used for the investigation of sulphide mineral-thio-collector system such as infra-red (IR) spectroscopy (Mielezarski and Yoon, 1989; Leppinen et aI., 1989; Persson et aI., 1991; Laajalehto et aI., 1993; Zhang., et aI., 2004a) and X-ray photoelectron spectroscopy (XPS) (Pillai et aI., 1983; Page and Hazell, 1989; Grano et aI., 1990; Laajalechto et aI., 1991). These surface sensitive spectroscopic techniques can be applied for the direct determination of the surface composition at the conditions related to flotation. 10

1.4

Interactions between Collector and Sulphide Minerals and Mixed Potential Model

75

(a)

~~

C E

t

7

;:$0

/1 .':

- - Buty I - - - - Ethyl ............ Methyl

.~1' 45 15

.:!

/1

11/...<.. .

,.

I."

...

{~·~····~~·.:::~:.::.::·.r.

-45

U~

..... , -

-75

I

I

I I

I I

(b)

...-- 80

- - Butyl - - - - Ethyl ............ Methyl

0

'-'"

bb 60 c: (j)

,.

ro

.A.

'0 40 ro

C 0

-.-

. .'

f ••••• ~ I.~· f ••••••

20

u

.

...... ~.

/

O'----~_____'__....L.___L___'-----'-'-.......----.-.-....~-----"'---'----'

-0.3

-0.2

-0.1 0.1 Potential (V, SHE)

0.2

0.3

Figure 1.5 Pyrite electrode at pH = 9.2 in 0.05 mol/L sodium tetraborate solution containing 1000 mg/L ofthree potassium alkylxanthates. (a) Cyclic voltammograms at 4 mV/s; (b) Contact angles measured after holding the electrode at each potential for 30s. Vertical lines are the reversible potential of the xanthate/dixanthogen couples (Gardner and Woods, 1977) 100

. . . --- - -- -r:.. . . --. -- - -II

,--

80

I I

...--

~ 60 /

W >

100

..,..,/

~-..,..,

/

,-------,/

/

<, \

\

60

\

\

\ \

\

20

\

I-------==~======~=----+-----~\

oo

\ \

~ 40 _ Prepared in

o

\

- - Prepared in oxidized condition

-0.5

-0.3

-0.1 0.1 Potential (V, SHE)

1: ~

8

\

reduced condition 20

-20

\

~ 5-

\

-60

\

0.3

0.5

-100

Figure 1.6 Anodic current and floatability of galena as a function of potential in the presence of ethylxanthate at pH = 8 after preparation in oxidizing and reducing environment (Guy and Trahar, 1985)

Leppinen (1990) used FTIR-ATR techniques to study the adsorption of ethyl xanthate on pyrite, pyrrhotite and chalcopyrite. The FTIR spectra of the reaction products on pyrite, pyrrhotite and chalcopyrite after treatment with 1xl 0- 4 mol/L KEX solution at pH= 5 is presented in Fig. 1.7. He found that the FTIR signals 11

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

of the xanthate species on pyrite occurred at approximately the same position as those of bulk dixanthogen together with a surface product, the signal of which coincided well with that of ferric ethyl xanthate. The adsorption product of KEX on pyrrhotite was much the same asthat on pyrite but no signal of iron xanthate species was observed on the surface of pyrrhotite. On chalcopyrite the signals of diethyl dixanthogen and a metal xanthate compound closely resembling copper ( I) xanthate were observed. These results led Leppinen (1990) to conclude that the main adsorption product of ethyl xanthate on pyrite and pyrrhotite is diethyl dixanthogen. An iron xanthate surface compound is also observed on pyrite at monolayer coverage. A mixture of copper xanthate compound and diethyl dixanthogen occurs on chalcopyrite.

1400

1300

1200

1100

1000

900

Wave number(crrr")

Figure 1.7 FTIR reflection spectra of pyrite, pyrrohotite and chalcopyrite after 15 min treatment with 10- 4 mollL KEX solution at pH = 5 (Leppinen, 1990)

1.5 Effect of Semiconductor Property of Sulphide Mineral on Its Electrochemical Behavior Most sulphide minerals are semiconductors as shown in Table 1.4. The difference of interactions of sulphide minerals with a collector at certain conditions is originated from their different semiconductor properties. Plaksin and Bessonov (1957) reported that p-type galena adsorbed xanthate more easily. Later, Eadington and Prosser (1969) investigated the effect of light irradiation on the semiconductor property of galena and its interaction with xanthate showing that the increase of the hole on the galena surface after light irradiation promoted the adsorption of xanthate. Esposito et al., 1987 pointed out that the pyrite in ore is p-type and the pyrite in coal is n-type giving rise to their different flotation behavior. Salvador and Tafalla (1991) and Mishra (1992) found that n-type pyrite-band was 12

1.5 Effect of SemiconductorProperty of SulphideMineral on Its Electrochemical Behavior

helpful for the reduction of oxygen. Richardson and Yoon (1993) revealed the change of the electronic structure (e.g. Femi energy level) of galena and pyrite surface with pH and the concentration of reagent and noted that the interaction between mineral and reagent depended on their energy match. Most importantly, the flotation behavior of sulphide minerals was reported to be able to be controlled by changing the surface electronic structure or energy level through light irradiation and other methods (Cata et aI., 1973; Cheng, 1999). Table 1.4 Semiconductor properties of different sulphide minerals

PbS

Width of forbidden band (eV) or conductivity 0.41

ZnS

3.6

(Zn,Fe)S·

0.49

HgS

2

Cu FeS2

0.5

< 0.1

FeS2

0.9

CoS2 CU2S

NiS2 CU2S

0.27

CoS

Meta conductor

2.1

FeS

Meta conductor

Minerals

Minerals Sb2S3 AS 2S3

Width of forbidden band (eV) or conductivity 1.72 2.44

Meta conductor

• The massfraction of Fe-w (Fe) 12.4%.

The authors (Sun et aI., 1991, 1993c, d, 1994b; Hu et aI., 2000; Wang et aI., 1992) used CNDO/2 method of quantum chemistry and band theory to study the flotation mechanism of sulphide minerals in the presence and absence of collectors. Figure 1.8 shows the energy level diagram of molecular orbital of galena, pyrite, common and activated oxygen and ethyl xanthate ion. The highest occupied molecular orbital (HOMO) of galena surface mainly consists of 3p orbital of sulphur atoms, and the lowest unoccupied molecular orbital (LUMO) mainly consists of the 6p orbital of the lead atom. There is a little overlap between the 6p orbital of the lead atoms and the 3p orbital of the sulphur atoms. It indicates that the Pb- S bond at the galena surface contains a large ionic bond character and a small covalent bond character. In the case of pyrite, the atomic orbital of iron and sulphur have equal contribution to the HOMO and LUMO. There is a great overlap between the atomic orbital of iron and sulphur, showing that Fe-S bond exhibits covalent bond character mostly and the Fe-Fe bond exhibits the metal bond character partly. As above, surface electron structure is much different between the mineral of the type galena and that of the type pyrite. Oxygen possesses the following molecular orbital

13

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals 16

EXLUMO

PbS

12 8 4

>' ~ ~

o -4

.u,

LUMO(rc)

.Ll,

LUMO(cr)* HOMO(1t)

-8 HOMO(1t) LUMO(1t)

.u,

-12

HOMO(rc)

-16

i

-l

.Ll;

HOMO(rc)

HOMO(1t*) LUMO

-201-24 '----

---l

Figure 1.8 Symmetry and energy of frontier molecular orbital (HOMO and LUMO)

In its two anti- 1[ orbital (1[*), there are two single electrons. The orbital can either accept or donate one electron, which can be either HOMO or LUMO. From Fig. 1.8, the energy values and symmetry of the frontier molecular orbital of galena, pyrite, HS-, EX-, and oxygen are shown in Table 1.5. Table 1.5 The energy values and symmetry of the frontier molecular orbital of galena, pyrite, HS-, EX- and oxygen Reactants Energy HOMO Symmetry EnergyLUMO Symmetry

Galena

Pyrite

Common oxygen

Activated oxygen

HS- ion

Ethyl xanthate ion

-7.757

-13.27

-15.82

-12.60

-11.70

-3.7

1t

1t

1t

1t

1t

1t

-3.133

-9.41

-15.82

-12.6

-8.7

12.1

1t

1t

1t

1t

(J'

(J'

1.6 Electrochemical Behaviors in Grinding System Grinding of sulphide minerals is essential for liberation in order to achieve effective flotation. Grinding processes, however, may also have a detrimental effect on flotation because grinding system is open to atmosphere. The interactions among the grinding media such as steel, active and noble minerals, and flotation reagents have a pronounced effects on the electrochemical reaction on sulphide surface and their flotation. There were a lot of investigations on the electrochemical behaviors in grinding system (Rao et aI., 1976; Hoey et aI., 1977; Guy and Trahar, 1984; Dai et aI., 2000). At first, the leakage, dislocation, and the incoming of impurities may be produced in a mineral crystal during grinding resulting in the change of surface properties of the mineral such as electron energy level and electrode potential. Rey and Formanek (1960) first reported the effect of grinding media on flotation. The steel media decreases the activity of sphalerite made the selective flotation 14

1.6

Electrochemical Behaviors in Grinding System

separation of galena with sphalerite. Thornton (1973) and Fahlstrom (1974, 1975) observed the improvement of flotation of copper ore in autogenous grinding than in carbon steel media. Rao et al. (1976) considered that the fine iron particles adhered to sulphide surface during grinding decreased the electrode potential and weakened the adsorption of xanthate resulting in the decrease of floatability of sulphide minerals. Harris (1988) found the worsen phenomena of sphalerite flotation due to the replacement of Fe2+ for Zn2+ during grinding. The compound containing iron on galena and sphalerite surface was detected by XPS. It was obvious that grinding changed the electrode potential and affected the flotation behavior of sulphide minerals. Table 1.6 summarizes the rest potential of several sulphide, sulfarsenide and arsenide minerals at near neutral pH. This table shows that the rest potential of individual minerals vary depending on the source and on the experimental conditions; nevertheless, those of different minerals are different. Table 1.6 Rest potential of sulphide, sulfarsenide and arsenide minerals of steel media at near neutral pH Minerals

Pyrrhotite

Sources Sudbury, Ontario Strathcona Mine, Ontario Stratheona Mine, Ontario Stratheona Mine, Ontario Stratheona Mine, Ontario Sudbury, Ontario Rouyn, Quebec

Distilled water

Rest potential (mV, SHE) References O2 N 2 Air 55 160 173 Adam et al. (1984)

Distilled water

125

262

295 Iwasaki et al. (1989)

Distilled water

155

290

335 Cheng (1991)

262

277

308

Solutions

0.001 mollL Na2S04 0.05 mol/L Na2S04 0.5 mol/L NaCI Distilled water Chalcopyrite 0.05 mol/L Messina, S.Africa Na2S04 Huanzala, Peru Pyrite

Coahuila, New Mexico Brushy Creek, Galena Missouri Unknown Arsenopyrite Unknown Cobaltite Synthetic Nickel Arsenide Synthetic

AISII020 AISII020

-

190 115

-

71 355 -

190 Li (1991) Adam et al. (1984) 371 Cheng (1991)

265 Li (1991)

Distilled water

405

445

485

0.001 mol/L Na2S04

389

391

393

Distilled water

142

172

218

Distilled water Distilled water Distilled water 0.001 mollL Na2S04

277 200 -90

303 275 5

323 303 97

152

154

157

AISI 1020 0.2% C Distilled water Mild steel

58

Distilled water 0.5 mol/L NaCI

Nakazawa and Iwasaki (1986)

-355 -255 -135 -515 -335 -175 -395 -

Pozzo and Iwasaki (1987) Nakazawa and Iwasaki (1985) Learmont and Iwasaki (1984) Iwasaki et al. (1989) Iwasaki et al. (1989) Iwasaki et al. (1989) Nakazawa and Iwasaki (1986) Pozzo and Iwasaki (1987) Adam et al. (1984) Adam et al.(1984)

15

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

With their rest potential much higher than that of the steel medium listed in Table 1.6, sulphide minerals act as a cathode while the steel medium acts as an anode. During grinding, minerals and grinding media (steel) come in repeated contact with each other, and galvanic current flows between the two surfaces of the sulphide minerals may be altered. Another important influence of grinding on the floatability of sulphide minerals is the galvanic interactions among the grinding media and different minerals, along with grinding and flotation atmospheres. Galvanic cells are set up by redox reactions taking place due to the difference in their rest potential. One of higher potential acts as a cathode, while that of lower potential as an anode. In grinding-flotation circuit of sulphide minerals, galvanic interactions exist between minerals and grinding media, usually steel, between mineral and mineral,' and among multiple mineral and grinding media (steel)system. The galvanic reactions are also governed by the mixed potential principle. The effect of galvanic interaction between the grinding media and a sulphide mineral on flotation has been studied with respect to pyrrhotite (Adam et aI., 1984; Nakazava and Iwasaki, 1985), galena (Learmont and Iwasaki, 1984), chalcopyrite (Yelloji Rao and Natarajan, 1989a and 1989b), and sphalerite (Yelloji Rao and Natarajan, 1989c).The balls undergo anodic dissolution whereas oxygen reduction could be the cathodic reaction at the surfaces of the sulphide minerals. The most prominent anodic reaction of steel ball would be the dissolution of iron: (1-7) Thus the dissolved ferrous species would further react with the hydroxyl ions generated at the cathodic sulphide mineral sits forming iron oxy-hydroxide species: (1-8)

t

t

Fe(OR)2 ~ FeOOR, Fe(OR)3

(1-9)

The cathodic reaction is oxygen reduction in Eq. (1-1). Because Fe(OR)3 and metal oxy-hydroxide species of iron, lead and zinc formed will coat the cathodic mineral surface, affecting its floatability. In the case of mineral-mineral interactions, a mineral with higher potential acts as a cathode, while a mineral with lower potential acts as an anode. For a multiple mineral/grinding media(steel)system. The galvanic interactions become more complex than the two-electrode systems. The galvanic reactions among multielectrode systems are also governed by the mixed potential principle as shown in an example of polarization curves involving pyrite, pyrrhotite and mild steel in Fig. 1.9 (Pozzo and Iwasaki, 1987). 16

1.6 Electrochemical Behaviors in Grinding System

-0.4

Py(26 cm-) !-----=------__ ~(l cm-) -

5J -0.5 u ir:

~

~

E Q) "0

0...

-0.6 -0.7 -0.8

-

-

-

-

<,

/

CD

"

1 /

<,

Mixed Potential

\

\

/

\

//tI /

- - PO l. em! -~ :.:: ~.-!' -

________ __--..- - -- - ® ~

-

I

I

I I

/'-

\

I

I

I corr (Py-MS)

/

/

/

@//

- \ - : , "/ <,

/

// 2 MS( 1 em )

? ,~

O-Po(26 em2 - ~ 'I ® I corr

. - - -'-', -..... I corr -. <. (Po-MS) Po: Pyrrhotite \, ", Py: Pyrite MS: Mild Steel \ \\

(Py-PoMS)

\

\

0.01

0.1 Current (rnA)

1.0

Figure 1.9 Determination of corrosion currents for pyrite-mild steel (Py-MS) pyrrhotite-mild steel (Po-MS) and pyrite-pyrrhotite-mild steel (Py-Po-MS)systems under abrasion in a quartzite slurry by adjusting to the surface area ratios of ground minerals and steel balls (Pozzo and Iwasaki, 1987)

The effect of galvanic interactions among sulphide minerals, such as chalcopyrite, galena and sphalerite in the presence and absence of a grinding ball material in the flotation of chalcopyrite, was studied by Rao and Natarajan (1989, 1990). They found that the floatability of chalcopyrite was affected significantly when the chalcopyrite contacted with active minerals (galena and sphalerite), either alone or together. Such an effect became more pronounced in the added presence of a grinding medium as well as oxygen. Nakazawa arid Iwasaki (1985) and Pozzo, Malicsi and Iwasaki (1988) investigated a pyrite-pyrrhotite contact and a pyrite-pyrrhotite-grinding media contact on flotation, respectively. They found that the floatability of pyrrhotite increased in the presence of pyrite, whereas it decreased in the presence of both pyrite and grinding media (mild steel). Similarly, a galvanic contact between nickel arsenide and pyrrhotite decreased the floatability of pyrrhotite (Nakazawa and Iwasaki, 1986). Wang and Xie (1990), reviewing the influence of grinding media and environment on flotation responses of sulphide minerals, concluded that grinding media and environment affected the surface properties and floatability of sulphide minerals mainly by electrochemical interaction, surface morphology and mechano-chemical reaction. Other important factors include surface area, solution composition, etc. To minimize galvanic interaction, it is necessary to choose an inert grinding atmosphere, such as ceramic or stainless steel mill balls and nitrogen aeration. Cases et al. (1990, 1991) studied the influence of grinding media on the adsorption of xanthate on galena and pyrite using XPS and FTIR. They found that only lead xanthate, either monocoordinated or in the three dimensional form 17

Chapter 1 General Review of Electrochemistry of Flotation of Sulphide Minerals

was formed on galena surface after dry grinding and that however, wet grinding led to heterogeneous adsorbed layer formed with monocoordinated lead xanthate and dixanthogen at low concentration; stoichiometric or non-stoichiometric lead xanthate, dixanthogen, and the dimmer of the monothiocarbonate at higher concentration. They also found that only dixanthogen was observed on pyrite surface after using stainless steel rods. Whereas, ferric iron xanthate and physically absorbed xanthate ions were formed when iron rods were utilized. A mixed polarization diagram (where the polarization behavior of the two different electrodes is represented) for the sphalerite-hypersteel combination is given in Fig. 1.10 (Vathsalaand Natarajan, 1989), in which the cathodic polarization curves for the sphalerite and the anodic polarization curves for the hypersteel ball material are seen to overlap. The active nature of the ball material is evident. The current values were observed to be lower in the absence of oxygen which indicated a lower anodic dissolution of the hypersteel grinding medium in the absence of oxygen. The presence of oxygen enhances galvanic interactions. Pozzo et al. (1990) found that the floatability of pyrite and pyrrhotite in the flotation of their mixtures were affected very little at neutral or acidic pH when the mixture was ground in a nitrogen atmosphere, whereas the recovery of pyrrhotite was greatly improved when the mixture was ground in an oxygen atmosphere. Therefore, the amount of oxygen used in flotation must be carefully controlled in order to alleviate the interactions among minerals. Trahar (1984) pointed out that the interactions among different minerals could be diminished if the minerals were ground separately and combined immediately before flotation, thereby demonstrating the effect of multi-electrode galvanic contacts.

800

~

u (/J

;>

a

~ -800

Hyper steel Sphalerite

02

• •

N2 0 l:::.

-1600 10

102

103

104

105

19l

Figure 1.10 Mixed polarization diagram involving hypersteel ball material in contact with sphalerite (unit of I: f.lA; Vathsala and Natarajan, 1989)

18

1.7 The Purpose of This Book

The separation of chalcopyrite and pyrrhotite at natural pH was reported to be possible when mild steel filings addition combined with magnetizing treatment were used under oxygen aeration (Cheng and Iwasaki, 1992). Mild steel filings adhere to the pyrrhotite surface by magnetic interaction. They form a localized galvanic cell involving oxygen reduction and the hydroxyl ion thereby generated reacts with ferrous ion from the pyrrhotite as well as from the mild steel filings and results in an iron hydroxide layer. FeS=Fe2+ +S -, ~S042Fe(OH)2 ~ Fe(OH)3 or Fe(OH)S04 2 Fe=Fe + + 2e-

(1-10)

/

(1-11)

The presence of oxygen enhances the formation of the surface coating and depresses the flotation of pyrrhotite. It appears, therefore, that although the floatability of individual mineral may be controlled by pulp potential, the presence of several sulphide minerals, particularly when they are ground with steel media, leads to galvanic interaction among them and the alteration of certain mineral surfaces may be accelerated. Then the pulp potential dependence of their floatability may not follow those of individual mineral.

1.7 The Purpose of This Book This book systematically summarizes the researches on electrochemistry of sulphide flotation in our group. The various electrochemical measurements, especially electrochemical corrosive method, electrochemical equilibrium calculations, surface analysis and semiconductor energy band theory, practically, molecular orbital theory, have been used in our studies and introduced in this book. The collectorless and collector-induced flotation behavior of sulphide minerals and the mechanism in various flotation systems have been discussed. The electrochemical corrosive mechanism, mechano-electrochemical behavior and the molecular orbital approach of flotation of sulphide minerals will provide much new information to the researchers in this area. The example of electrochemical flotation separation of sulphide ores listed in this book will demonstrate the good future of flotation electrochemistry of sulphide minerals in industrial applications.

19

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

Abstract This chapter first explains the natural flotability of some minerals in the aspect of the crystal structure and demonstates the collectorless flotaiton of some minerals and its dependence on the Eh and pH of pulp. And then the surface oxidation is analysed eletrochemically and the relations of Eh to the composition of the solutions are calculated in accordance with Nemst Equation. The Eh-pH diagrams of several minerals are obtained. Thereafter, electrochemical determination such as linear potential sweep voltammetry (LPSV) and cyclic voltammetry (CV) and surface analysis of surface oxidation applied to the sulphide minerals are introduced. And recent researches have proved that elemental sulfur is the main hydrophobic entity which causes the collectorless flotability and also revealed the relation of the amount of sulfur formed on the mineral surfaces to the recoveries of minerals, which is always that the higher the concentration of surface sulphur, the quicker the collectorless flotation rate and thus the higher the recovery. Keywords natural flotability; collectorless flotation; Eh-pH diagram; linear potential sweep voltammetry; cyclic voltammetry; XPS; UV

2.1

Crystal Structure and Natural Floatability

The structure of galena is shown in Fig. 2.1. Galena is a common and popular mineral among rock hounds. The structure of galena is identical to that of halite, NaCI, which has face-centered cubic structure in the crystal, each Pb atom is bonded to 6 8 atoms and each 8 atom is bonded to 6 Pb atoms. It have perfect cleavage in [001], [010] and [100] direction. Pyrite is cubic with the octahedrally coordinated metal atoms at the comers and face centers of the cube unit cell (see Fig. 2.2). "Dumb-bell" shaped disulphide atoms 8~- lie at the center of the cube and at the midpoints of the cube edges. The mid-point of the 8 2 group occupies the CI sites of the NaCI structure, while the Fe atoms occupy the Na positions. The S~- pairs are oriented such that their axes are parallel to four non-intersecting bodies' diagonals of the cubic space lattice and give pyrite hemihedral symmetry. Each S atom is coordinated to three Fe atoms and one ·8 atom in a distorted tetrahedral configuration. Therefore, the cleavage planes for both galena and pyrite exhibit hydrophilic characteristics due to their structural feature.

2.1

Crystal Structure and Natural Floatability

(a)

(b)

Figure 2.1 Crystal structure of galena (a) Side view of galena crystal; (b) Top view of galena crystal

(a)

(b)

Figure 2.2 Crystal structure of pyrite (a) Pyrite (100) plane; (b) Showing the coordination of ions

In the layered structure of molybdenite (MoSz), each atom is surrounded by six sulfurs at the vertices of a trigonal prism. The prism is linked by their edges into a continuous layer within which the centers of half of the prisms are occupied by cations as shown in Fig. 2.3. That is, a molybdenite crystal consists of S-Mo-S sandwich-like layers held together by van der Waals bonds. During comminution, fracture occurs by the breakage of these weak residual bonds. Thus the breakage faces are naturally hydrophobic. The stibnite has a chain structure (Vanghanand Craig, 1978). It appears that the sulphur-antimonybonds are "weak", at least weaker than most metal-sulphur bonds and the surface fracture faces are somewhat non-polar in nature. Orpiment has a layer structure consisting of corrugated pyramidal AsS3 groups linked via common S atoms. Realgar has a 21

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

ring structure, having a puckered ring molecule (Vanghan and Craig,1978). The fracture of these two minerals occurs by the rupture of weaker residual bonds and hence they will also display natural hydrophobicity. Minerals such as molybdenite, orpiment, realgar and stibnite, hence, are easily floated without the need of collectors or of special conditions for collectorless flotation, i.e. no specific Eh is required for flotation, nor is there any display of variable floatability with Eh • Such mineral exhibit is called "natural floatability" induced by their structural features. Figure 2.4 shows that molybdenite is highly floatable. The flotation recovery is basically independent of Eh , i.e. no specific Eh is required for the flotation of molybdenite. It has been reported that the flotation recovery of orpiment and realgar can reach to 100% without collector (Finkelstein and Goold, 1972).

(a)

(b)

Figure 2.3 Layer structured molybdenite (MoSz) (a) Side view of molybdenite; (b) Top view of molybdenite 120 100 ~

80

~

'6 '"o0 '"

r

.,

..

0

MoSz

60

;>

~

•

pH=8.2

40

• .. Na-S

• NaZSZ0 4 • (NH4hSzOg

20

0'------'--------'--------'------' -0.2

0

0.2 0.4 Potential s;(Y, SHE)

0.6

Figure 2.4 Floatability-potential curves for molybdenite in the absence of collector (Wang et al., 1993a)

22

2.2

Collectorless Flotation

2.2 Collectorless Flotation 2.2.1 Effect of Pulp Potential on Flotation at Certain pH Most sulphide minerals do not exhibit natural floatability. They have, however, collectorless floatability in specific pulp potential ranges, under the condition of which the sulphide mineral surface has been rendered hydrophobic by surface self-oxidation. Many other authors indicated that collectorless flotation of sulphide minerals can occur only under moderately oxidizing environments and be controlled by pulp potential (Gardner and Woods, 1979; Hayes et aI., 1987, 1988; Sun, 1990; Wang et aI., 1991a,b,c,d; Cheng and Iwasaki, 1992; Zhang et aI., 2004a,b,c,d,e,f). The collectorless floatability of chalcopyrite has been studied in some detail and some results are shown in Fig. 2.5 (Guy and Trahar, 1985; Wang, 1992). It has been found that there is a clear distinction between flotation and non-flotation, which appear to be pH and E h dependent. The upper limit and lower limit of pulp potential for collectorless flotation of chalcopyrite change with pH. 100 80

PEO (mg/L)

-10 --400

~

~ 60 C Q) >

0

o Q)

• pH=4.0

o pH=2.5

40

• pH=IO.2 l:l. pH=ll • pH=8

~

20

0'--------£.lF-"----'------"'----~---'--------'

-0.4

-0.2

0 0.2 0.4 PotentialEPt (V,SHE)

0.6

0.8

Figure 2.5 Floatability-potential curve for chalcopyrite in collectorless flotation (Guy and Trahar, 1985; Wang, 1992)

The influences of pulp potential on the floatability of galena, jamesonite, pyrite, pyrrhotite, sphalerite and marmatite are, respectively given in Fig. 2.6, Fig. 2.7 and Fig. 2.8. It can be seen that these minerals show collectorless flotation in certain range of potential and exhibit different upper and lower potential limit for flotation. In Fig. 2.6, the potential range of collectorless flotation is 0.15 - 0.3 V for galena at pH = 6 and 0.45 - 0.6 V for jamesonite at pH = 4.7. Pyrrhotite exhibits good collectorless floatability in the potential range of 0.25 - 0.35 V at pH = 8.8, but pyrite shows weak collectorless flotation, the range of potential is around 0.17 V at pH = 6 as seen in Fig. 2.7. The potential range of collectorless flotation is 0.45 - 0.55 V for marmatite and 0.15 - 0.25 V for sphalerite at pH= 4.7. 23

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals 100.---------------------. ___ Galena -+- Jamesonite(pH=4.7) 80 ~

60

~

'6 Q)

> o

~ 40 20

o

100

200 300 400 500 PotentialEPt(mV, SHE)

600

700

Figure 2.6 Effect of pulp potential on collectorless flotation behaviors of galena and jamesonite 100.---------------------. ____ Pyrite -+- Pyrrhotite(pH=8.8) 80

~ 60

'6 Q)

> o u

~ 40

20

o

100

200 300 PotentialEPt (mV, SHE)

400

Figure 2.7 Effect of pulp potential on collectorless flotation behaviors of pyrite and pyrrhotite

2.2.2 Pulp Potential and pH Dependence of Collectorless Floatability Figures 2.6, 2.7 and 2.8 provided the evidence that there exists the criticalupper and lowerlimitof pulppotential for collectorless flotation at certain pH. Figure 2.9 further demonstrated the flotation recovery of jamesonite as a function of potential at different pH. It is obvious thatjamesonite has very good collectorless floatability in different potential range, which much depended on different pH. The 24

2.2 Collectorless Flotation 100..----------------------.

--- Sphalerite -+- Marmatite(pH=4.7) 80

~ 60

'6 (l)

;>

oo

40

~

20

o

100

200

300 400 500 Potential EPt (mV, SHE)

600

700

Figure 2.8 Effectof pulp potential on collectorless flotation behaviors of sphalerite and marmatite 100,....--------------------, - 0 - pH=2.2 90 - - 0 - pH=4.7 80 ---6- pH=8.8 ~pH=12.1 ~ 70 ~ 60

'6 ~ o

50

~ 40 30 20

10

o

200

400

600 800 Eh(mV, SHE)

1000

1200

Figure 2.9 Effectof pulppotential on collectorless flotation behaviors of jamesonite at differentpH

floatable potential region located more oxidized atmosphere in acid medium and toward lower potential region in alkaline medium. If we take recovery to be 50% as the floatable and non-floatable criterion, Floatability- potential curve at certain pH for sulphide minerals can be generally plotted as Fig. 2.10. Define the upper limit potential (ED), below which recovery is greater than 50% and lower limit potential (E L ) , above which recovery is greater than 50%, The upper limit potential and lower limit potential of some minerals at different pH can be determined through the curves obtained from flotation tests. The E~, E~ and E~ , E~, respectively, expressed the upper and lower limit of potential determined using platinum and mineral electrode. It has been reported that

25

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

platinum and pyrite electrodes behaved almost identically and the same curve was obtained when the floatability was plotted against pyrite electrode and platinum electrodes (Heyes and Trahar, 1984). However the formation of oxidized surface layers on mineral electrodes through reaction with dissolved species in flotation pulps affected the responses of mineral electrodes. Some investigators found that pulp potential in the same system can be different for different indicator electrodes (Labonte and Finch, 1988; Rand and Woods, 1984).

(-)

0 Potential

(+)

Figure 2.10 General floatability-potential curve

Nevertheless the potential to be monitored in flotation system would be the mineral potential, not the solution potential. The electrode constructed from the mineral being concentrated should be the most appropriate electrode for E h measurements because the relevant E h is established at the mineral/solution interface (Woods, 1991). Figure 2.11 to Fig. 2.13 show the flotation Eh-pH area of sulphide minerals determined from their floatability-potential curve at various pH using mineral electrode. It can be seen from the Figures that there exists an Eh-pH area where the collectorless flotation of sulphide minerals is possible. Figure 2.11 and Fig. 2.12 show that jamesonite and pyrrhotite exhibit wider potential and pH range for collectorless flotation, especially in strong acidic media they exhibit a wider potential range for collectorless flotation. The upper limit potential E~ and the lower limit potential E~ of pyrrhotite collectorless flotation are, respectively, 750 mV and 450 mV in strong acidic medium, and 300 mV and 50 mV in strong alkaline medium. The upper limit potential E~ and lower limit potential E~ of jamesonite collectorless flotation are, respectively, 820 mV and 400 mV in strong acidic medium, and 300 mV and 100 mV in strong alkaline medium. With the increase of pH, the potential of collectorless flotation ofpyrrhotite andjamesonite decreases. Figure 2.13 shows that marmatite covers a wide range of potential for collectorless flotation only in strong acidic pH region.

26

2.2

Collectorless Flotation

900 800 700 G3' 600 ~ 500 ~"' 400

--.- Fe-Eh(u) -

Fe-Eh(1)

~ 300 200 100

o

2

4

6

pH

10

8

12

14

Figure2.11 Eh-pH area of collectorless flotation of pyrrhotite

900r - - - - - - - - - - - - - - - - - - - , 800 -+- Pb-Eh(u) - - - Pb-Eh(1)

700 ~ 600 ~ ir: 500

>"'

S

400

~ 300 200 100

o

4

2

6

10

8

12

14

pH

Figure2.12 Eh-pH area of collectorless flotation ofjamesonite

900 800

--*- Fe-Eh(u)

700

~

Fe-Eh(1 )

~ 600 :c ~"' 500

5

~

400 300 200 100

o

2

4

6

8

10

12

14

pH

Figure2.13 Eh-pH area of collectorless flotation of marmatite

27

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

There are some other reports on collectorless flotation of chalcopyrite, galena, pyrite and arsenopyrite showing that chalcopyrite possesses very strong collectorless floatability. The potentialrange of collectorless flotation of chalcopyrite is wider in wider pH range. Galena exhibits wider potential and pH range for collectorless flotation, Arsenopyrite exhibits a wider potentialrange for collectorless flotation in weak acidic media. Pyrite floats over a narrow range of potential in strong acidic pH (Sun, 1990; Sun et aI., 1992).

2.3

2.3.1

Electrochemical Equilibriums of the Surface Oxidation and Flotation of Sulphide Minerals The Surface Oxidation of Sulphide Minerals and Nernst Equation

It appears that an oxidizing potential is usually a requirement for collectorless flotation. The surface products of oxidation will clearly have an influence on surface hydrophobicity and hence on flotation. The initial oxidation of sulphides in acid solutioncorresponds to a reactionof the type MS=Mn + +S+neI1GO

= I1GoM + 110°S -I1GoMS n+

(2-1a) (2-1 b)

where I1Go is the standardfree energyabove reaction. The oxidation of sulphides corresponds to a reaction of the type in neutral or alkaline solution (2-2a) (2-2b) Alternative oxidation reactions mayleadto the production of oxy-sulphur species (2-3a) (2-3b) (2-4a) (2-4b) The nature of the oxidation products will in general determine the subsequent 28

2.3

Electrochemical Equilibriums ofthe Surface Oxidation and Flotation of Sulphide Minerals

flotation behavior. It seems reasonable to assume then that the hydrophobic entity is sulphur itself produced by the reactions similar to (2-1) or (2-2), the potential of which defines the lower limit of potential at which the collectorless flotation of sulphide minerals begins. However, if oxidation is by reactions (2-3) or (2-4), collectorless floatability would not be expected. The potential corresponding to reactions (2-3) or (2-4) defme the upper limit of potential at which the collectorless flotation of sulphide minerals ceased. Therefore, the potential relations of surface oxidation of sulphide minerals to produce elemental sulphur on flotation and the potential dependence of oxidation to produce thiosulphate on depression may be obtained. The oxidation-reduction potential or redox potential (Eh) is a measure of the tendency of a solution to be oxidizing or reducing. Oxidation and reduction are basically electrical processes that are readily measured by an electrode potential. All measurements are referred to the standard hydrogen electrode, the potential 2 of which is taken as 0.00 V at 298 K, the H2 pressure as 101325 N/m (1 atm) and activities of H2 and H+ as unity. When the half-cell reaction is written as an oxidation reaction: Reduced state = Oxidized state + nElectrons The relationship between any redox potential and the standard electrode potential is given by the Nemst equation:

E; = EO + RT In [Oxidizedstate] = 2.303RT 19 [Oxidizedstate] nF

[Redued state]

nF

(2-5)

[Reduced state]

where R is the gas constant (8.314 J·K-1·mol-I ) , Tis absolute temperature (K), F l is the Faraday constant (96490 C'mol- ) , and n is the number of electrons involved in the redox reaction. EJ is the electrode potential that is related to standard free energy as:

dGO EO=-nF

(2-6)

The [oxidized state] and [reduced state] are, respectively, the various items in the right and left of the reactions like Eqs. (2-1a) to (2-4a). dd can be calculated using various standard free energy dates reported from literature (Vanghan and Craig, 1978; Garrels and Christ, 1965; Wang and Hu, 1989). For reactions at 298 K, (2.303RT/F) = 0.059, so that

- dGg 0.059 [Oxidized state] Eh - - -98+ - - 1g - - - - nF n [Reduced state]

(2-7)

In the light of Eq. (2-7), the potential of surface oxidation of sulphide minerals to form elemental sulphur or oxy-sulphur species could be calculated. 29

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

2.3.2

Electrochemical Equilibriums in Collectorless Flotation

Although the nature of the hydrophobic entity responsible for the self-induced flotation of sulphide minerals remains somewhat obscure, most reported results clearly show that it is only when the environment becomes slightly oxidizing that flotation is observed. The elemental sulphur and polysulphide-intermediates in the oxidation of sulphide to sulphur have ever been suggested to be of the hydrophobic species. Whatever it is, there is no doubt that sulphur can generate hydrophobicity and floatability. For chalcopyrite, the sulphur-producing reactions include: in alkaline medium (2-8a) (2-8b) where the date of ~G~e(OH)3 ' ~G~us ,~G~20 can be taken from literature (Vanghan and Craig, 1978; Garrels and Christ, 1965; Wang and Hu, 1989), and ~G~uFes2 can be calculated according to the reaction ofEq. (2-9): (2-9a)

Ksp =

10-61.5

~GO' = ~G~e2+ + ~G~u2+ + ~G~2- - ~G~uFes2 =-RTlnKsp = 350.0 1(kJ/mol)

(2-9b)

(2-9c)

~G~UFes2 = ~G:e2+ + ~G~u2+ + 2~G~2_ - ~Go' = -84.935 + 64.978 + 2 x 91.881- 350.01 = -187.106(kJ/mol)

(2-9d)

thus ~GO

= -694.544 - 48.953 + 187.106 + 3 x 237.19 =155.179(kJ/mol) (2-10a)

EO = !1Go = 155.l79xlOOO =0.536(V) nF 3x96490

(2-10b)

The potential-pH relationship ofEq. (2-8a) is

E; = 0.536 - 0.059pH

(2-10c)

Similarly, in weak acidic medium

30

CuFeS2 = C u S + Fe 2+ +S+2e-

(2-11a)

~Go = 53.2(kJ/mol)

(2-11b)

2.3

Electrochemical Equilibriums of the Surface Oxidation and Flotation of Sulphide Minerals

EO = O.276(V)

(2-11c)

E; = 0.276+ 0.0295Ig[Fe 2+]

(2-11d)

The formation of sulphur on chalcopyrite surface would be expected to occur at 0.15 V at pH= 6 by assuming [Fe2+] = 10-4 mollL according to reaction (2-8), 0.08V at pH = 8 and -0.1 V at pH = 11 according to reaction (2-9). While flotation does begin very near these potential for the experimental conditions reported by some authors as shown in Table 2.1. Table 2.1 flotation Minerals

Relationship between initial potential producing elemental sulphur and Chalcopyrite

Galena PbS

Ehit (V)

Ehie (V)

e; (V)

CuFeS2 E hie (V)

pH=6

0.24 (Eq. (2-12b))

0.26 (Sun, 1990)

0.16 (Eq. (2-11b))

0.18 (Feng, 1989)

pH=8

0.28 (Eq. (2-13b))

0.27 (Guy and Trahar, 1984)

0.08 (Eq. (2-10b))

0.08 (Trahar, 1984)

pH= 11

0.1 (Eq. (2-14b))

0.16 (Sun, 1990)

-0.11 (Eq. (2-10b)) -0.1 (Trahar, 1984)

Ehit is the theoretical potential calculated on the basis of electrochemical equilibrium; Ehie

is the experimental valuesreported by someauthors.

For galena, the reactionsproducing elemental sulphurmay include in acidic media PbS =Pb 2+ +S+2e-

l1d = 68.315(kJ/mol) EJ = 0.354(V) { t; = 0.354+ 0.0295Ig[Pb2+] = 0.236(V)

(2-12a) (2-12b)

in weak alkaline media PbS+2H 20 =Pb(OH)2 +S+2H+ +2e-

(2-13a)

O

I1G = 146.085(kJ/mol) {

EO = 0.757(V)

(2-13b)

E; = 0.757- 0.059pH

in alkaline media PbS+ 2H20 =

HPbO; +S+3H+ +2e-

(2-14a)

O

I1G = 228.102(kJ/mol) {

EO = 1.182(V)

r;

(2-14b)

=1.064-0.0885pH 31

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

The reaction potential producing elemental sulphur are 0.24 V at pH= 6 based on the reaction (2-10), 0.28 V at pH = 8 on the reaction (2-11), and 0.1 V at 4 pH= lion the reaction (2-12) with 10- mol/L concentration of dissolved species. The reported flotation initial potential (see Table 2.1) are very close to these theoretical calculation values. The theoretical and experimental values in Table 2.1 indicate that the elemental sulphur might be responsible for the hydrophobicity of sulphide surfaces. At different pH media the formation of elemental sulphur occurs and hence the flotation behavior undergoes different processes.

2.3.3

Eh-pH Diagrams of Potential and pH Dependence of Flotation

Eh-pH diagrams have become a standard method for illustrating equilibrium relationships between dissolved and solid species and have been applied successfully to investigate flotation systems (Hepel and Pomianowski, 1977; Chander et al., 1979; Pritzker and Yoon, 1984a,b; Johnson and Munro, 1988). In the following calculations all the thermodynamic data are from Garrels and Christ (1965) and Vanghan and Craig (1978). The Eh-pH diagrams of surface oxidation of sulphide minerals are constructed based on the reactions below, assuming the concentration 4 of all dissolved species to be 10- mol/L and considering elemental sulphur as a metastable phase because thiosulphate and even sulphate may be formed but frequently at potential higher than the calculated equilibrium potential, i.e. at over potential (1]) varying magnitudes (take 1]= 0.5 V in subsequent calculation, Peters, 1977, 1986).

1. Eh-pH Diagram of Chalcopyrite Besides reactions (2-8) and (2-11), the reactions, which could produce sulphur on chalcopyrite, may involve the progressive oxidation of the products of these two reactions CuS as following: CuS =

Cu2+ + S + Ze

(2-15a)

~GO = 113.858(kJ/mol)

EO == 0.59(V)

(2-15b)

e, ==0.59+0.0295Ig[Cu 2+] CuS + 2H 20 =

CU(OH)2 + S + 2H+ + 2e-

(2-16a)

~Go = 166.44(kJ/mol)

EO == 0.862(V)

E; == 0.862 - 0.059pH 32

(2-16b)

2.3 Electrochemical Equilibriwns of the Surface Oxidation andFlotation of Sulphide Minerals

At certain high potential, chalcopyrite surface is further oxidized, and thus the reactions producing hydrophilic species are: 2CuFeSz +3H zO =

2CuS+ 2Fez+ +szoi- + 6H+ + 8e-

(2-17a)

I1Go = 285.768(kJ I mol) EO=O.37(V)

(2-17b)

t; = 0.331- 0.044pH 2CuS+3H zO=2Cu z+ +szoi- +6H+ +8e-

(2-18a)

I1Go = 407.229(kJ/mol) EO = 0.528(V)

(2-18b)

Eh = 0.44 - 0.044pH

2CuFeSz +9HzO=2CuS+2Fe(OH)3+SZ0~- +12H+ +10e-

(2-19a)

I1Go = 489.278(kJ/mol) EO = 0.507(V)

(2-19b)

Eh = 0.48-0.071pH 2CuS+ 7H zO =

I1Go

2Cu(OH)z +szoi- + 10H+ + Se

= 512.247(kJ/mol)

EO = 0.664(V) E;

(2-20a)

(2-20b)

= 0.635 - 0.0738pH

The Eh-pH diagram of chalcopyrite is constructed combining the Eqs. (2-8), (2-11), and (2-15) to (2-20) as given in Fig. 2.14, where the Eh limits of water stability and other unimportant species are not considered for the sake of simplification. It can be seen from Fig. 2.14 that the upper limit and the lower limit potential of collectorless flotation of chalcopyrite at various pH values well agree with the potential defined respectively by reactions producing thiosulphate and elemental sulphur. The Eh-pH area where chalcopyrite possesses self-induced floatability is just the region where metastable elemental sulphur exists, indicating that elemental sulphur is a hydrophobic entity. When above the upper limit potential, hydrophilic species such as thiosulphate and metal hydroxides are formed, and the flotation ceases. Figure 2.14 also demonstrates that chalcopyrite exhibits the best self-induced floatability, which has wider upper and low limit potential of flotation at almost all pH regions. 33

Chapter 2

Natural Floatability and Collectorless Flotation of Sulphide Minerals 1.0 .---~-----.....-----------,

CU2++S20~- :

0.8 0.6

U3' 0.4 :r:

I I

"

Cu2++S

"~"

0

~I

,,

- - - - - - - - - - , - - - ..l... _

~ 0.2 Cu2++Fe2++S ~ 0 CuFeS2· •

"~

o • Feng, 1990

-0.2

•

~

Sun, 1990

o • Trahar, 1984

-0.4 ~_--L--_----L...._-----'_ _ o 2 4 6 8 pH

....L....-_-L--_-----L._----'

10

12

14

Figure 2.14 Electrochemical phase diagram for chalcopyrite with elemental sulphur as metastable phase. Equilibrium lines (solid lines) correspond to dissolved species at 10- 4 mol/L. Plotted points show the upper and lower limit potential of collectorless flotation of chalcopyrite reported from Sun (1990), Feng (1989) and Trahar (1984)

2. Eh-pH Diagram of Galena For galena, the reactions not producing hydrophobic species are 2PbS + 3H 20 =

2Pb 2+ + S20;- + 6H+ + Be

(2-21a)

I1Go = 316.101(kJ/mol)

EO = 0.409(V)

(2-21b)

E; = 0.32 - 0.044pH 2PbS+7H20=2Pb(OH)2+S20;- +10H+ «se ~Go

(2-22a)

= 471.643(kJ/mol)

EO = 0.611(V)

(2-22b)

s, = 0.581- 0.0737pH 2PbS + 7H 20 =

I1Go

2HPbO; + S20;- + 12H+ + 8e-

(2-23a)

= 635.675(kJ/mol)

EO = 0.823(V)

(2-23b)

E; = 0.734 - 0.0885pH The Eh-pH diagram of galena is constructed through the reactions (2-12) to (2-14) and (2-21) to (2-23) and presented in Fig. 2.15. It may be seen from Fig. 2.15 that the lower limit potential of collectorless flotation of galena at various pH are well defined by the conditions producing elemental sulphur. The upper limit potential of self-included collectorless flotation of galena at various pH is 34

2.3

Electrochemical Equilibriums ofthe Surface Oxidation and Flotation of Sulphide Minerals

1.0 I

Pb2++S 20j-

0.8

N~

0N tr:

+ IN

0.6

0

.D

c,

~ 0.4

:J:

sr:

-;;: 0.2

~

=c

•

Pb2++S PbS

0 ~

-0.2

•

• Guy and Trahar, 1984 0 SUO, 1990

-0.4 2

0

4

6

8

10

12

14

pH

Figure 2.15 Eh-pH diagram for galena in aqueous solutions with elemental sulphur as metastable phase. Equilibrium lines correspond to dissolved species at 10- 4 mollL. Plotted points show the upper and lower limit potential of collectorless flotation of galena reported from Sun (1990, 1992) and from Guy and Trahar (1984)

determined by the reactions producing thiosulphate, lead hydroxide and plumbate. The Eh-pH area where galena has self-induced collectorless floatability is just the region where metastable elemental sulphur exists, indicating that metastable elemental sulphur is a hydrophobic entity. Galena appears to have better collectorless floatability at wider pH and potential range although it shows less floatablive than chalcopyrite.

3. Eh-pH Diagram of Pyrite and Arsenopyrite Pyrite oxidation reactions include the reactions producing hydrophobic species: FeS2 = F e 2+ +2S+2e-

(2-24a)

~Go = 65.689(kJ/mol)

E O = 0.34(V)

(2-24b)

Eh = 0.34 + 0.059Ig[Fe 2+] = 0.24(V) FeS2 + 3H 2 0 = ~Go

Fe(OH)3 + 2S + 3H+ + 3e-

(2-25a)

=167.603(kJ/mol)

EO = 0.579(V)

(2-25b)

E; = 0.579 - 0.059pH and the reactions producing hydrophilic species: (2-26a) 35

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

I1GO = 244.891(kJ/mol) {

EO = 0.423(V)

(2-26b)

Eh = 0.344 - 0.059pH

FeS2 + 6H 20 =

Fe(OH)3 + S20~- + 9H+ + 'le:

(2-27a)

I1GO = 347.021(kJ/mol)

EO = 0.514(V)

(2-27b)

{ E = 0.48 - 0.076pH h

For arsenopyrite, oxidation reactions include the reactions producing elemental sulphur: FeAsS+ 4H 20 =

Fe2+ + H3As04 +8+5H+ +7e-

Eo = O.303(V)

(2-28b)

{ E; = 0.236 - 0.042pH FeAsS+4H 20 =

(2-28a)

Fe2+ + H2AsO~ +S+6H+ +7e-

Eo = 0.333(V) { E; = 0.266 - 0.0506pH

(2-29a) (2-29b)

FeAs8+7H20=Fe(OH)3+HAsO~- +8+10H+ +8e-

Eo= 0.4771(V)

(2-30a) (2-30b)

{ E = 0.448 - 0.0738pH h and the reaction producing thiosulphate: 2FeAsS+IIH 20=2Fe 2+ +2H 3As04 +S20~- +16H+ +18e-

Eo= 0.339(V)

(2-31b)

{ E; = 0.273 - 0.052pH

°;- +18H+ +18e-

2FeAs8+11H 20=2Fe2+ +2H2AsO~ +82

Eo= 0.362(V) { E; = 0.296 - 0.059pH 2FeAsS + 17H20 =

2Fe(OH)3 + 2HAsO~- + S20~- + 26H+ + 20e-

Eo= 0.475(V) { i; =0.416-0.0767pH 36

(2-31a)

(2-32a) (2-32b) (2-33a) (2-33b)

2.3 Electrochemical Equilibriums of the Surface Oxidation and Flotation of Sulphide Minerals

The Eh-pH diagrams of surface oxidation of arsenopyrite and pyrite are shown in Fig. 2.16 and Fig. 2.17, respectively. Figure 2.16 shows that Eh-pH area of self-induced collectorless flotation of arsenopyrite is close to the area forming sulphur. The reactions producing elemental sulphur determine the lower limit potential of flotation. The reactions producing thiosulphate and other hydrophilic species define the upper limit of potential. In acid solutions arsenopyrite demonstrates wider potential region for collectorless flotation, but almost nonfloatable in alkaline environment. It suggests that the hydrophobic entity is metastable elemental sulphur. However, in alkaline solutions, the presence of 1.0

Fe2++S20J-:

0.8

H2As04"

"I,

0

~~

0rrJ

1'\

I

0.6

$

::r:

0.4

>"

0.2

~I

01 00,

-< ::c

\0

~I

Fe 2++S

C/)

~

'\

::CI 1

I

•

0

FeAsS

-0.2 -0.4

0

2

4

6

8

10

12

14

pH

Figure 2.16 Eh-pH diagram for arsenopyrite in aqueous solutions with elemental

sulphur as metastable phase. Equilibrium lines correspond to dissolved species at 10- 4 mollL. Plotted points show the upper and lower limit potential of collectorless flotationof arsenopyrite reportedfrom Sun (1990, 1992) 1.0 0.8 0.6

$

::c

0.4

>" '-"

0.2

C/)

~

0 -0.2 -0.4

0

2

4

6

8

10

12

14

pH

Figure 2.17 Eh-pH diagram for pyrite in aqueous solutions with elemental sulphur as metastable phase. Equilibrium linescorrespond to dissolved species at 10- 4 mol/L.

Plotted points show the upper and lower limit potential of collectorless flotation of pyrite reportedfrom Sun (1990, 1992) 37

Chapter 2

Natural Floatability and Collectorless Flotation of Sulphide Minerals

metastable elemental sulphur does not render arsenopyrite hydrophobic and exhibits collectorless floatability. It may be attributed to the presence of Fe(OH)3 and HAsO;- at the same time on the arsenopyrite surface. The hydrophobicity of elemental sulphur is not sufficient to overcome the hydrophilicity by Fe(OH)3 and HAsO;-. Therefore, it might be supposed that the relative amounts of Fe(OH)3, HAsO;- and sulphur and accordingly the rations of hydrophobic to hydrophilic species perhaps determine the floatability of arsenopyrite. Figure 2.17 shows that although the metastable elemental sulfur may be present at pyrite surface, pyrite does not exhibit self-induced collectorless floatability except in very strong acidic media and a narrow oxidized Eh range. Such behavior may be similar to that of arsenopyrite in alkaline solutions due to the formation of hydroxides, thiosulphate.

4. Eh-pH Diagram of Jamesonite The Eh-pH diagram of jamesonite is constructed combining the Eqs. (2-34) to (2-49) as following:

2+ Pb4FeSb6S14=3Sb2S3 + 4Pb 2+ + Fe + 5S + 1Oe

(2-34a)

EJ = 0.4841(V) { E = 0.4841 + 0.00591g[Fe2+] [Pb2+]4 = 0.3213(V)

(2-34b)

h

Pb4FeSb6S14+ 12H20=6HSb02+ 4Pb 2+ + Fe2+ + 14S + 18H++ 28e- (2-35a)

EJ = 0.4690(V) {

Eh = 0.4690 + 0.00211Ig[Fe2+] [HSb0 2]6[Pb2+]4 - 0.0379pH

= 0.3447 -

(2-35b)

0.0379pH

Pb4FeSb6S14+ 18H20=6SbO; + 4Pb 2+ + Fe2+ + 14S + 36H++ 40e-

(2-36a)

EJ = 0.5317(V) E; = 0.5317 + 0.00 148Ig[SbO;]6[Pb2+]4[Fe2+] - 0.0531pH

(2-36b)

= 0.4445 - 0.0531pH

Pb4FeSb6S14+21H20=6SbO;+4Pb2+ +Fe(OH)3+14S+39H+ +41e- (2-37a)

EJ = 0.5445(V) Eh = 0.5445 + 0.00 144Ig[SbO;]6[Pb2+]4 - 0.0561pH = 0.4683 - 0.0561pH

38

(2-37b)

2.3 Electrochemical Equilibriums of the Surface Oxidation and Flotation of Sulphide Minerals

Pb4FeSb6S14+42H20=6SbO; +4Pb2+ +Fe(OH)3+7S20~- +81H+ +6ge- (2-38a) if=0.5120(V) Eh= 0.5120+ {

0.0008551g[Pb2+]4[SbO;]6[S20~-]7 - 0.0693pH

(2-38b)

= 0.4359-0.0693pH

Pb4FeSb6S14+77H20=6SbO;+4Pb2++Fe(OH)3+ 14S0~- +151H++125e- (2-39a) if = 0.4165(V) E h = 0.4165 + 0.0004721g[Pb2+]4[SbO;]6[SO~-]14 - 0.0713pH

(2-39b)

= 0.3594- 0.0713pH Pb4FeSb6S14+ 12H20=6HSb02+ 4PbS+ Fe2+ + lOS + 18H+ + 20e- (2-40a)

f if = 0.5150(V)

LE = 0.5150+ 0.002951g[Fe +] [HSb0 h

2

2]

6- 0.0531pH = 0.4049- 0.0531pH (2-40b)

Pb4FeSb6S14+15H20=6HSb02+4PbS+Fe(OH)3+10S+21H+ +21e- (2-41a) if = 0.5405(V)

, { E = 0.5405 + 0.002821g[HSb0 - 0.0592pH = 0.4521 - 0.0592pH h 2]6

(2-41b)

Pb4FeSb6S14+15H20=6HSb02+4Pb2+ +Fe(OH)3+14S+21H+ +2ge- (2-42a) if = 0.4893(V) {

E h = 0.4893 + 0.002041g[HSb02]6[Pb2+]4 - 0.0427pH= 0.3813 - 0.0427pH

(2-42b)

Pb4FeSb6S14+21H20=6SbO; +4PbS+Fe(OH)3+10S+39H++33e- (2-43a) if = 0.5907(V) {

(2-43b)

Eh = 0.5907+ 0.001791g[SbO;]6 - 0.0697pH = 0.5346- 0.0697pH

Pb4FeSb6S14+29H20=6SbO; +4Pb(OH)2+Fe(OH)3 +14S+47H++41e- (2-44a) if = 0.6231(V) { Eh = 0.6231 + 0.001441g[SbO;]6 - 0.0676pH= 0.5780- 0.0676pH (2-44b) Pb4FeSb6S14+ 50H20=6SbO; + 4Pb(OH)2 + Fe(OH)3 + 7S20~- + 89H+ + 6ge(2-45a) 39

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

EJ = 0.5588(V)

{ Eh = 0.5588 + 0.000855Ig[8bO;]6[820;-]7 -

0.0761pH = 0.5012 - 0.0761pH (2-45b)

Pb4Fe8b6814+85H 20=68bO; +4Pb(OH)2+Fe(OH)3+1480;- +159H++ 125e(2-46a)

{

EJ = 0.4443(V) E h= 0.4443 + 0.000472Ig[8bO;]6[80;-]14 - 0.0750pH = 0.3974 - 0.0750pH (2-46b) Pb4Fe8b6814+ 29H 20=68bO; + 4HPbO; + Fe(OH)3 + 148 + 51H++ 41e(2-47a)

EJ = O.7059(V)

{E

h=

0.7059 + 0.00 144Ig[HPbO;]4[8bO;]6 - 0.0734pH = 0.6297 - 0.0734pH (2-47b)

Pb4Fe8b6814+ 50H 20=68bO; + 4HPbO; + Fe(OH)3 + 78 20;- + 93H++ 6ge(2-48a)

EJ = 0.6080(V) {

Eh = 0.60~0 + 0.000855Ig[HPbO;]4[8bO;]6[8 20;-]7 - 0.0795pH

(2-48b)

= 0.5319 - 0.0795pH

Pb4Fe8b68 14 + 85H20=68bO;+4HPbO; + Fe(OH)3 + 1480;- + 163H++ 125e(2-49a)

EJ = 0.4715(V) {

Eh = 0.4715 + 0.000472Ig[HPbO;]4[8bO;]6[80~-]14 - 0.0769pH

(2-49b)

= 0.4144- 0.0769pH

The Eh-pH diagram for jamesonite is given in Fig. 2.18. It may be seen from Fig. 2.18 that in acid and neutral solutions, the reactions producing elemental sulphur may render jamesonite surface hydrophobic and jamesonite shows good collectorless flotation. In alkaline solution, the hydrophilic species Fe2+, Pb 2+, H8b02, SbO;, are produced, the collectorless floatability of jamesonite becomes weaker, it may be attributed to the presence of Fe(OH)3 and Pb(OH)2 at the same time on the jamesonite surface. The relative amounts of hydrophobic sulphur and hydrophilic Fe(OH)3 and ~b(OH)2 perhaps determine the collectorless floatability ofjamesonite.

40

2.4 Electrochemical Determination of SurfaceOxidationProducts of SulphideMinerals 1.0

SbO]"

Pb2+

Fe3+

0.8

S20j-

N

£

0

SbO]"

':5 0.6

0-

SbO]" SbO]" S Pb2+

> ~

SbO)"

HPb0 2

S20j-

Fe(OHh

S 0.2

S20jN

SbO)"

Pb4FeSb6S 14

i' 0

':5

-0.2

-0.4

0-

CD HSb0 2+Fe2++Pb2++S ® HSb02+Fe2++PbS+S ® HSb0 2+Fe(OHh+PbS+S

0

@ Sb03"+Fe(OH)3+PbS+S @ Sb2S3+Fe2++Pb2++S 0

2

4

6

8

10

12

14

pH

Figure 2.18 Eh-pH diagramfor jamesonitein aqueous solutions with elemental sulphur as metastable phase. Equilibrium lines correspond to dissolvedspeciesat 10-4 mol/L

2.4

Electrochemical Determination of Surface Oxidation Products of Sulphide Minerals

Many investigators have used different techniques to study the electrochemical behavior of different sulphide mineral electrodes in solutions of different compositions. Linear potential sweep voltammetry (LPSV), and cyclic voltammetry (CV) have been perhaps, used most extensively and applied successfully to the investigation of reactions of sulphide minerals with aqueous systems. These techniques have provided valuable information on the extent of oxidation as a function of potential for various solution conditions and have allowed the identity of the surface products to be deduced. The oxidation of chalcopyrite has been studied in some detail (Gardner and Woods, 1979; Biegler and Home,1985; Guy and Trahar, 1985; Kelsall and Page, 1985; Pang and Chander, 1990). Figure 2.19 presents the voltammogram of chalcopyrite electrode at pH = 11. It is evident that the anodic peaks occur due to the anodic oxidation of chalcopyrite. The potential at which the anodic current rises are in reasonable agreement with those calculated based on reaction (2-8) at pH = 11. It suggests that the formation of sulphur would be expected to occur at - 0.12 V at pH = 11 by reaction (2-8). 41

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

o l.(')

pH=ll

-0.6

-0.4

-0.2

0

0.2

0.4

Potential (V, SHE)

Figure 2.19 Linear potential sweep voltammograms for chalcopyrite electrode. Scan rate at 10 mV/s

In Fig. 2.19, on the reverse scan a cathodic peak was observed at about -0.27 V owing to the reduction of the products of anodic process. (2-50a)

{

~GO

=12.6(kJ/mol)

EO = -0.065(V)

(2-50b)

E; = 0.053 - 0.0295pH

The potential calculated using above reaction is -0.27 V at pH = 11 at dissolved 4 species 10- mol/L corresponding to the potential of the cathodic current- rise, further indicating the elemental sulphur as the oxidation product. Voltammograms obtained with galena (see Fig. 2.20) show that the initial potential of anodic oxidation of galena is around 0.15 V, which is very close to the potential of formation of elemental sulphur in Eqs. (2-12) and (2-13) and corresponding to the initial potential of flotation beginning of galena in Fig. 2.6 suggested that the oxidation product of galena surface is sulphur. The potential at which the anodic current and anodic oxidation of galena increased evidently is around 0.4 V, which is close to the potential according to reactions (2-21) and (2-22) considering elemental sulphur as a metastable phase. It indicates that the further oxidation of galena formed hydrophilic oxy-sulphur species or hydroxides and galena collectorless flotation ceased as shown in Fig. 2.6. It has also been reported that the initial product of oxidation was elemental sulphur at all pH with Pb2+ being formed in acid and Pb(OH)2 and HPbO~- in alkaline media (Hamilton and Woods, 1984; Lamacha et aI., 1984; Pritzker and Yoon, 1984a,b; Sun et aI., 1993a,b,c,d). The voltammogram for jamesonite is given in Fig. 2.21. It can be seen that there are four obvious anodic peaks at the first cycle voltammetry. The peak current of ap., ap, and CP2 constantly increase with the number of cycle adding, but the peak height of ap, and ap, decreases at the second cycle and then keeps constant even by multi-cycle according to the experimental appearances. It was 42

2.4

Electrochemical Determination of Surface Oxidation Products of Sulphide Minerals

0.0002 0.0001

l'

0.0000 I I I I I I

~

~ -0.0001

:E2

-0.0002

I I I

-0.2

0.0

0.2

0.4

0.6

0.8

E(Y) Figure 2.20 Voltammograms for galena at natural pH (sweep rate: 30 mV/s)

assumed that apt peak corresponds to the reaction of forming the deficient-metal and sulfur-rich compound, whereas ap2 is related to the reaction in which the deficient-metal sulphide is further oxidized into elemental sulfur. As described in the following, the Eh values of the reactions (2-51), (2-52) are unanimous with the peak potential in Fig. '2.21. 600 400 200 ,-....

0

N

E o

<:

-200

~ -400 ~

pH=6.86 - Cycle lth - - Cycle 5th

-600 -800 -1000 -1.0

-0.5

0.0

E(Y)

0.5

1.0

Figure 2.21 Multi-voltammograms for jamesonite electrode in 0.1 mol/L KN03 mixed phosphate buffer solution at pH = 6.86 and scanning rate of 50 mVIs

Pb4FeSb6S 14+ 12H20=6HSb02+4PbS+Fe2+ + 10S+ 18H++20e- (2-51a)

~(f = 994.02(kJ/mol) E>=0.5150(V) Eh = 0.5150 + 0.00295Ig[Fe

2+]

(2-51b) [HSb0 2] 6 - 0.0531pH

= 0.4049 - 0.0531pH = 0.04063(V)

(pH = 6.86) 43

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

ap2: Pb4FeSb6S14+ 12H20=6HSb02+4Pb2+ + Fe2+ + 14S+ 18H++28e- (2-52a)

~d = 1267.22(kJ/mol)

t> = 0.4690(V)

Eh = 0.4690 + 0.00211Ig[Fe2+][HSb02]6[Pb2+]4 - 0.0379pH

= 0.3447 - 0.0379pH= 0.08447(V)

(2-52b)

(pH = 6.86)

The peak of ap, corresponds to the reaction of the formation of s2oi-. The change of apa peak may be the result of the over-potential of SO~- by the following reactions. Although the E h values of the reactions (2-53), (2-54) are smaller, they occur at the higher potential due to the existing over-potential of S20;-, SO~ ions, respectively. apj: Pb~eSb6S14+33H20=6HSb02+4Pb2+ +Fe2+ +7S 2032-+60H++ 56e(2-53a) ~d = 2522.48(kJ/mol)

t>= 0.4668(V)

Eh = 0.4668 + 0.00106Ig[Fe 2+] [HSb02]6[Pb2+]4[S2032-]7 - 0.0634pH

= 0.3661- 0.0634pH = -0.06903(V)

(2-53b)

(pH=6.86)

apa: Pb~eSb6S14+68H20=6HSb02+4Pb2+ +Fe2+ + 14S042-+ 130ft+ I 12e(2-54a)

~d = 4161.62(kJ/mol)

t> = 0.3850(V)

(2-54b) 2+][HSb02]6[Pb2+]4[S042l14 Eh = 0.3850 + 0.000529Ig[Fe - 0.0685pH = 0.3179 - 0.0685pH= -0. 1519(V)

CPl:

(pH = 6.86)

Fe(OH)2 + H20=Fe(OH)3 + H++ e

t> = 0.271(V) { s; = 0.271- 0.059pH

(2-55a) (2-55b)

The cyclic voltammetry for sphalerite electrode is presented in Fig. 2.22. It follows from Fig. 2.22 that the potential range of collectorless flotation of sphalerite is 155 - 270 mv, For the collectorless flotation of sphalerite, the lower limit of flotation corresponds to the following reactions:

44

ZnS~S+Zn2+ +2e-

(2-56a)

t> = 0.311(V)

(2-56b)

ZnS+2H20~S+Zn(OH)2 +2H+ +2e-

(2-56c)

t> = 0.645(V)

(2-56d)

2.4 Electrochemical Determination of Surface Oxidation Products of Sulphide Minerals 0.0015 0.0010 -..

0.0005

~
0.0000

~

-0.0005

E :i

-0.0010 -0.0015 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

E(V, SHE)

Figure 2.22 Voltammograms for sphalerite electrode in naturalpH valueat scanning rate of 0.50 mV/s (0.1 mol/L KN03)

The potential is 155 mV based on reaction (2-56a) and 280 mV based on reaction (2-56c), with 10-6 mollL concentration of dissolved species, the initial reaction of sphalerite correspond to reaction (2-56a). The further oxidation of sphalerite surface are: 2ZnS+3H20~s2oi- +2Zn

2+ +6H+ +8e-

.eO=0.174(V)

(2-57a) (2-57b)

2ZnS+7H20~s2oi- +2Zn(OH)2 +10H+ +8e-

(2-58a)

.eO = 0.345(V)

(2-58b)

Considering the formation of s2oi- needing 0.5 V over-potential, assuming the concentration of all dissolved species to be 10-6 mol/L, the reaction potential are 0.28 V and 0.37 V respectively, corresponding to reactions (2-57) and (2-58), the upper limit potential of flotation of sphalerite depends on reaction (2-57). Pyrrhotite is one of many sulphides which display collectorless flotation resulting from the formation of sulphur on the mineral surface (Hamilton and Woods, 1981; Heyes and Trahar, 1984; Hodgson and Agar, 1984). The anodic scan section of cyclic voltammetry for pyrrhotite electrode in pH = 2.2, 4.7, 7.0, 8.8, 11, 12.1, 12.7 buffer solutions respectively, is presented in Fig. 2.23. The cyclic voltammograms curve at pH = 8.8 is also presented in Fig. 2.23. It can be seen from Fig. 2.23 that anodic current peak emerges at about -0.1-0 V when pH < 11. As pH increases, the peak moves to the left. This peak may correspond to the following reactions: (2-59a) (2-59b) 45

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

-0.2

0

0.2

0.4

0.6

E(V, SHE)

Figure 2.23 Voltammograms for pyrrhotite electrode in 0.1 mol/L KN0 3 mixed buffer solutionof different pH at scanning rate of 10 mV/s

Due to the formation of S on the surface of pyrrhotite, the collectorless flotation of pyrrhotite can be carried out. When pH= 12.7, the anodic peak disappeares, and the anodic current increases rapidly. It indicates that only the oxidation of pyrrhotite takes place on the surface of pyrrhotite. So the flotation of pyrrhotite couldnot occur. Thesephenomena may correspond to the following reactions. (2-60a) (2-60b) Otherreports also demonstrate the potential of pyrrhotite flotation commences is corresponding to the initial potential of anodic oxidation process explained in terms of the following reaction by Hamilton and Woods (1981, 1984), Heyes and Trahar(1984). (2-61a) (2-61b) 46

2.4

Electrochemical Determination of Surface Oxidation Products of Sulphide Minerals

Pyrite and arsenopyrite have similar oxidation and self-induced collectorless flotation behavior. It is generally suggested that anodic oxidation of pyrite occurs according to reactions (2-24) in acidic solutions (Lowson, 1982; Heyes and Trahar, 1984; Trahar, 1984; Sun et aI., 1991; Chander et aI., 1993). The oxidation of pyrite in basic solutions takes place according to reactions (2-25). Since pyrite is flotable only in strong acidic solutions, it seems reasonable to assume that reaction (2-24) is the dominant oxidation at acidic solutions. Whereas pyrite oxidizes to oxysulfur species with minor sulphur in basic solutions.

0.0002 0.0001

<'E

0.0000

~ -0.0001

I I

I I I

:E2

-0.0002

I I I

-0.2

0.0

0.2

0.4

0.6

0.8

E(V, SHE)

Figure 2.24 Voltammograms for pyrite electrode in natural pH at scanning rate of 0.50 mV/s (0.1 mollL KN0 3)

Figure 2.24 shows the voltammograms of pyrite electrode in neutral media. It follows that the potential range of flotation relates to reactions (2-24) and (2-25). The potential calculated in reaction (2-24) is 161 - 180 mY, pyrite exhibit collectorless floatability due to the formation of elemental sulphur. The oxidation of arsenopyrite has some more arguments. Kostina and Chemyak (1979) concluded that the reaction was significant only in caustic soda solution and that the adsorption of hydroxide ion was a key initiating step in the oxidation reaction. Beattie and Poling (1987), Sanchez and Hiskey (1988) and Feng (1989) considered that the oxidation of arsenopyrite was observed only under basic conditions by a reaction of the type: FeAsS+ IIH20~Fe(OH)3+ HAsO~- + SO~- + 18H+ + 14e-

(2-62)

Dunn et al. (1989) attributed the oxidation of arsenopyrite electrode in acid to the reaction of type: (2-63a)

47

Chapter 2

Natural Floatability and Collectorless Flotation of Sulphide Minerals

EO=0.388(V)

(2-63b)

{ E; = 0.329 - 0.0443pH

The reaction product was identified as a-sulphur using XRD and SEM analysis. Sanchez and Hiskey (1991) reinvestigated the oxidation reaction of arsenopyrite and a two-step reaction sequence was suggested by the electrochemical measurements. The initial step was described by (2-64) The second step involved the oxidation of the arsenite and sulfur according to the following reactions: H 2AsO; + H 20 =

HAsO~- + 3H+ + 2e-

S+4H20=SO~- +8H+ -

ee

(2-65) (2-66)

Because arsenopyrite is floatable in acidic conditions and non-floatable in basic conditions (see Fig. 2.16), it seems reasonable to assume that reactions (2-63) or (2-28) and (2-29) are dominant oxidation in acidic solutions. Elemental sulphur is responsible for the hydrophobicity of arsenopyrite in acidic media. In alkaline solutions, reactions (2-64) and (2-65) may be dominant resulting in the formation of oxy-sulfur species and arsenate species with minor sulphur.

2.5

Surface Analysis of Oxidation of Sulphide Minerals

Although the results of flotation, Eh-pH diagram and voltammetry studies show the evidence that the hydrophobic entity responsible for the collectorless flotation of most sulphide minerals is sulphur. The attempts to identify sulphur on the surface of sulphide minerals and to relate the amount produced to their floatability have not produced unequivocal results. Luttrell and Yoon (1984a,b) were unable to establish a relation between surface coverage and floatability. They suggested that the hydrophobic entity might be copper polysulphide rather than sulphur. Buckley et al. (1985) maintained that the absence of any detectable elemental sulphur in the X-ray photoelectron spectrum (XPS) of sulphide minerals indicated that the surface oxidation of sulphide minerals involves progressive removal of metal atoms from the sulphur lattice leaving metal-deficient sulphides with sulphur lattices little altered from the original structure. They also found that when the potential was made more positive in the oxidant solutions than in air-saturated solutions, the metal-deficient layer decomposed to form elemental sulphur. Chen et al. (1986) detected the presence of elemental sulphur on galena surface in the XPS at pH = 6 and Eh = 0.45 Y. It can be seen from Fig. 2.25 that S2-, Sand 80;are present with binding energy of 161 eY, 163 eVand 167 eYe

48

2.5

Surface Analysis of Oxidation of Sulphide Minerals

PbS

150

155

160

165

170

175

E(eV)

Figure 2.25 S (2p) XPS of floated concentrate of galena in the absence of collector pH = 6, E h = 0.4 V (Chen et aI., 1986)

Despite the conflicting evidence, Heyes and Trahar (1984) believe there is sufficient evidence to confirm the presence of sulphur on mineral surface. They leached the surface of floated pyrrhotite from a typical test with cyclohexane and have examined the leach solution in a UV spectrophotometer. They found that sulphur could be extracted from the surface of pyrrhotite, which had been floated in the absence of collector. As can be seen from Fig. 2.26, the spectrum from the leached pyrrhotite was compared with the spectrum of sulphur dissolved in cyclohexane indicating that sulphur was present at the surface. Kelebek and Smith (1989) used UV spectrophotometer to determine sulphur in the ethanol extract from the surface of floated galena and chalcopyrite showing that the amount of sulphur on the minerals can be correlated with their flotation rate which was found to be first order within the critical surface tension range.

220

240

260 280 Wave length(nm)

300

320

Figure 2.26 Comparison of spectra of cyclohexane extracts of floated pyrrhotite with that of sulphur dissolved in cyclohexane (Heyes and Trahar, 1984)

The correlation between the amount of extracted sulphur and floatability was further investigated. Figure 2.27 represents the relationship between the recovery of marmatite, pyrrhotite and jamesonite and the amount of extracted sulphur at

49

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

pH = 4.7. From Fig. 2.27 it can be seen that the recovery of sulphide minerals corresponds well to the amount of extracted sulphur from their surface. The more the amount of S extracted is, the greater the recovery of sulphide minerals. When the amount of S attain a certain value, the flotation recovery of minerals is the maximum. Therefore it can be concluded that S may be responsible for the hydrophobicity of sulphide minerals in collectorless flotation. 100...------------------, 90 80 ,.-.....

70

~

60

8

40 30

~;> 11)

~

50

-+- Marmatite -

20

-6-

10

o

0.02 0.04 Amount of S (mg/g)

Pyrrhotite Jamesonite

0.06

0.08

Figure 2.27 The relationship between the recovery of marmatite, pyrrhotite and jamesonite and the amount of extracted sulphur

The collectorless flotation recovery of marmatite, pyrrhotite and jamesonite and the amount of extracted sulphur as a function of pH are shown in Figs. 2.28, 2.29 and 2.30. Figure 2.28 shows that the trend of recovery of marmatite is consistent with change of the amount of extracted sulphur. They all decrease with the increase of pH. In acidic pH media, both collectorless flotation recovery of marmatite and the amount of extracted sulphur from its surface are the maximum. 100 0.020 80

-0-

Recovery

- - Amount of S

'i: '6 ;> Q)

~

~

S

60

0.010 ~ o

Q)

8

0.015

t:::s

40

o 0.005 E

~

<e:

20 0.000

o

2

4

6

pH

8

10

12

14

Figure 2.28 Collectorless flotation recovery of marmatite and the amount of extracted sulphur as a function of pH 50

2.5

Surface Analysis of Oxidation of Sulphide Minerals

100

0.05

•

80

0.04 '@

0.03

• Recovery ____Amountof S

t: ::s

o

-0-

o

v: o

t.+-

0.02 20

g~

0.01

E

<

0.00 14

'----''''------'---L--'---'-~---'-_.....__'______'__~~__'____'

2

4

6

pH

8

10

12

Figure 2.29 Collectorless flotation recovery of pyrrhotite and the amount of extracted sulphur as a function of pH 100

0.07 0.06

80

0.05 '@ ~

~

'6

60

0.04

u (1)

"

g

rJ:1 't.+-

(1)

> 0

~

0.03 ~

40

e

::s

0 0.02 E

- 0 - Recovery --- Amountof S

20

0.01

<

0.00 14

'-----'---L'--'---'----'---"----'-----'----L--~~...6.._"_____'

o

2

4

6

pH

8

10

12

Figure 2.30 Collectorless flotation recovery of jamesonite and the amount of extracted sulphur as a function of pH

However, Fig. 2.29 and Fig. 2.30 show that the trend of recovery of pyrrhotite and jamesonite are not so well consistent with that of the amount of extracted sulphur. At lower pH region, the lower value of extracted 8 is required for collectorless flotation of jamesonite and pyrrhotite. At higher pH region, the greater amount of extracted 8 is required for good flotation of jamesonite and pyrrhotite. In strong alkaline medium (pH> 11), although there is enough amount of extracted 8, the recovery ofjamesonite and pyrrhotite still decline, which may be due to the hydrophilicity of OH-. Sun et al. (I993c; I994a,b) reported that the potential range where galena is able to be induced flotation is in excellent agreement with the range where sulphur could be extracted. The flotation recovery of galena and the amount of extracted sulphur occurs at a plateau in the range of potential 0.45 to 0.65 V. The self-induced flotation recovery of galena is increased with the increase of the amount of extracted sulphur. If we take the amount of extracted sulphur [8] at recovery 50% as the critical value of [8] required for self-induced collectorless 51

Chapter 2 Natural Floatability and Collectorless Flotation of Sulphide Minerals

flotation at various pH. There are different recoveries vs. [S] curves in different pH and hence different critical sulphur amount as shown in Fig. 2.31. It may be seen from Fig. 2.31 that the lower the pH and the lower the critical sulphur value are required. The linear relations between [S]c and pH were obtained for galena as in the following equation. [S], = a + bpH

(mg/g)

(2-67)

which shows the balance between hydrophobicity rendered by sulphur and hydrophilicity induced by OH-. 0.02 r---------~----,

High EPt area

o .... 2

4

6

pH

8

10

12

14

Figure 2.31 Critical value (at recovery> 50%) of the amount of extracted sulphur as a function of pH in collectorless flotation of galena

It may be concluded from the discussion above that the collectorless flotation was observed for many sulphides in moderately oxidizing potential region, but not in strongly reducing potential region. The available evidence suggests that the hydrophobic entity could be sulphur produced by superficial oxidation of the mineral. In general, flotation is observed in the potential-pH areas where elemental sulphur is metastable. There is significant agreement between the lower potential boundary of the flotation region and the potential at which the anodic current begins in a potential sweep. The amount of extracted sulphur on the sulphide minerals can be correlated with their collectorless flotation behaviors. The higher the concentration of surface sulphur, the faster the collectorless flotation rate and thus the higher the recovery.

52

Chapter 3 Collectorless Flotation in the Presence of Sodium Sulphide

Abstract The sodium sulphide-induced collectorless flotation of several minerals are first introduced in this chapter. The results obtained are that sodium sulphide-induced collectorless flotation of sulphide minerals is strong for pyrite while galena, jamesonite and chalcopyrite have no sodium sulphideinduced collectorless flotability. And the nature of hydrophobic entity is then determined through Eh-pH diagram and cyclic voltammogram, which is element sulphur. It is further proved with the results of surface analysis and sulphur-extract. In the end, the self-induced and sodium sulphide-induced collectorless flotations are compared. And it is found that the order is just reverse in sodium sulphide-induced flotation to the one in self-induced collectorless flotation.

Keywords

sodium sulphide-induced collectorless flotation; Eh-pH diagram

3.1 Description of Behavior The flotation results of three sulphide minerals in the presence of sodium sulphide are presented in Fig. 3.1 as recovery-concentration curves. In contrast to selfinduced collectorless flotation, pyrite and arsenopyrite are strongly floatable while chalcopyrite and galena are weakly floatable in the presence of sodium sulphide, 100 80 ,,-...,

~

'6

60

o

40

(l)

> 0 (I)

~

20

o

50

100 150 200 Na2S concentration (mg/L)

250

Figure 3.1 Flotation recovery of sulphide minerals as a function of Na2S concentration at pH = 11 (Wang and Hu, 1992; Wang and Long, 1991)

Chapter 3

Collectorless Flotation in the Presence of Sodium Sulphide

It can be seen from Fig. 3.1 that pyrite is strongly floatable with recovery above 90%. Chalcopyrite and galena are not floatable. At pH = 11, the recovery of chalcopyrite is sharply decreased with the increase of the concentration of Na2S and exhibits no floatability. The recovery of galena is also sharply decreased when the concentration of Na2S is greater than 40 mg/L and also exhibits poor sulphur-induced collectorless flotation. In other words, the self-induced collectorless flotation of chalcopyrite and galena is depressed by the addition of sulphide. Alternatively, the recovery of pyrite is evidently increased when the concentration of Na2S added is above 2xl0-4 at pH = 11, showing strong Na2S induced collectorless floatability. 100 80 ,-,

'$. 60

'6 Q)

>

0

(,) Q)

0:::

40

__ pH=2.2 pH=4.7 -..- pH=8.8 ....... pH=12.1 -4-

20

o

2 4 6 8 Na2S concentration (10-3 mol/L)

10

Figure 3.2 Sodium sulphide-induced collectorless flotation recovery of pyrrhotite as a function of Na-S concentration

The effect of Na2S concentration on flotation recovery of pyrrhotite at different pH was present in Fig. 3.2. It can be seen from Fig. 3.2 that at pH= 4.7 and 8.8 the recovery of pyrrhotite have almost not changed with the increase of the concentration of sodium sulphide. It suggests that Na2S concentration may not affect the self-induced collectorless flotation of pyrrhotite. At pH=2.2, the recovery of pyrrhotite decreases with the increase of concentration and when the concentration of Na2S is greaten than 3 xI 0-3 mol/L the recovery of pyrrhotite is sharply decreased, indicating the depressing action of Na2S on the self-induced collectorless flotation of pyrrhotite. At pH = 12, the addition of Na2S improves slightly the self-induced collectorless flotation of pyrrhotite. Flotation recovery ofjamesonite as a function of Na2S concentration is shown in Fig. 3.3. It follows that the recovery ofjamesonite is decreased by the addition of low dosage of Na2S at various pH. In contrast to self-induced collectorless flotation, the collectorless flotation of jamesonite is depressed in the presence of sodium sulphide. In alkaline pH range, the self-induced collectorless flotation of jamesonite is sharply depressed by the addition of Na2S. In acidic pH range, the 54

3.1

Description of Behavior

influence of Na2S on the self-induced collectorless flotation of jamesonite is much less than in alkaline media. 100~----

-4-

pH=2.2 pH=4.7

pH=7 pH=8.8 ....... pH=12.1

20

o Figure 3.3

2 4 6 8 Na2S concentration(10-3 mol/L)

10

Flotation recovery ofjamesonite as a function of Na-S concentration

The effect of Na2S concentration on flotation recovery of marmatite at different pH value is presented in Fig. 3.4. It can be seen from Fig. 3.4 that the collectorless flotation of marmatite is depressed by the addition of Na2S at acidic condition. The lower pH, the stronger the depress effect. The addition of Na2S has no effect on the flotation of marmatite in strong alkali. In neutral and weak alkali media, the addition of low dosage of Na2S can improve the flotation of marmatite, but when the concentration of Na 2S added is above lxl0-3 mollL, the flotation of marmatite is also depressed. 100~--------------'

80

~

C OJ o> ~

cG

60

40 20

o

2 4 6 8 Na2S concentration(10-3 mol/L)

10

Figure 3.4 Sulphur-induced collectorless flotation recovery of marmatite as a function of Na2S concentration

Replot the date points on three minerals at pH= 8.8 and pH= 12.1 from Fig. 3.2 to Fig. 3.4 is shown in Figs. 3.5 and 3.6. The collectorless floatability difference 55

Chapter3 Collectorless Flotationin the Presenceof SodiumSulphide

of three minerals in the presence of sodium sulphide is clearly seen in descending order of pyrrhotite, marmatite and jamesonite. At pH = 8.8, the collectorless 3 flotation recovery when the concentration is 2 x 10- mol/L is, respectively, 72% for pyrrhotite, 60% for marmatite and 30% for jamesonite. At pH= 12.1, the collectorless flotation recovery when the concentration is 2 x 10-3 mol/L is, respectively, 90% for pyrrhotite, 10% for marmatite and 10% for jamesonite. It indicates the possibility of collectorless flotation separation of these three minerals in the presence of sodium sulphide. 100.--------------------, ~ Pyrrhotite 90 Jamesonite 80 ----6- Marmatite ,-., 70 'cf( 60

'6 ~

50

8 40 ~ 30 20 10

o

2

4 6 8 10 Na2S concentration(10- 3 mol/L)

12

14

Sulphur-induced collectorless flotation recovery of pyrrhotites, jamesonite and marmatite as a functionof Na-S concentration at pH = 8.8

Figure 3.5

100 90 80 70 ,-.,

'cf(

60

Q)

50

o Q)

40

'6 > 0

0:::

~

----6-

Pyrrhotite Jamesonite Marmatite

30 20 10

o

2

4 6 8 10 Na2S concentration(I 0- 3 mol/L)

12

14

Sulphur-induced collectorless flotation recovery of pyrrhotite, jamesonite and marmatiteas a function of'Na-S concentration at pH = 12.1

Figure 3.6

The influences ofpulp potential on the Na2S -induced flotation ofpyrite at pH = 8 was examined by Heyes and Trahar (1984) for an initial sodium concentration of 10-3 moIIL. The region 'Of fairly strong Na2S -induced floatability over a 'wider potential range between 0 and 0.5 V was reported. They noted that the floatability

56

3.2 Nature of Hydrophobic Entity

of pyrite was much reduced compared with self-induced collectorless flotation for essentially similar conditions. They believe that this result was related to the rates of oxidation of the pyrite samples which were from different sources. Guy and Trahar (1984) found that as with chalcopyrite there was no flotation in the presence of free hydrosulphide ion and the galena ground in sodium sulphide solution exhibited some floatability over a similar potential range to that in self-induced flotation, but began at more reducing potential and recovery was relatively lower. The results above show that the sodium sulphide-induced collectorless floatability of sulphide minerals is strong for pyrite. Galena,jamesonite and chalcopyrite have no sodium sulphide-induced collectorless floatability. Marmatite and pyrrhotite showed some sodium sulphide-induced collectorless floatability in certain conditions.

3.2

Nature of Hydrophobic Entity

It was established by Tolun and Kitchener (1963-1964) that a platinum electrode became hydrophobic when hydrosulphide ion (HS-) was oxidized at the electrode surface. A contribution to the hydrophobicity may be the production of sulphur. According to thermodynamics, the hydrolysis reaction of sodium sulphide is Na2S+ 2H20=H2S + 2NaOH

(3-1)

H2S=HS- +H+

(3-2a)

K al = 10- 7.02

(3-2b)

HS-=H+ +S2-

(3-3a)

10- 13.9

(3-3b)

Ka2 =

The redox reactions of species S2-, HS- and H2S in solution and Eh-pH relations are (assuming the total concentration of dissolved sulphur species to be 10-3 mol/L): (3-4a) O

E = 0.157(V ) { E = 0.157 - 0.059pH h

HS- + 4H20=SO~- + 9H++ 8e-

(3-4b) (3-5a)

EO= 0.252(V) { E = 0.252 - 0.066pH h H2S + 4H20=SO~- + 10H++ 8e-

(3-5b) (3-6a) 57

Chapter3 Collectorless Flotationin the Presenceof SodiumSulphide

{ EO = O.303(V) Eh= 0.303- 0.074pH HzS + 4H zO=HSO; +9H+ +8e-

{ EO =O.289(V) Eh= 0.289 - 0.066pH HzS=S + 2H+ + 2e-

{ EO= O.1418(V) Eh = 0.2303- 0.059pH HS-=S+H+ +2e-

{ If =-O.065(V) Eh= 0.0235- 0.0295pH SZ-=S+2e{ If=-O.4761(V) Eh=-0.3876(V) S + 4HzO=SO~- + 8H+ + 6e-

{ If =O.3572(V) Eh= 0.3276- 0.07867pH S+4HzO=HSO; +7H+ +6e-

{ If = O.3384(V) Eh= 0.3088- 0.0688pH

(3-6b) (3-7a) (3-7b) (3-8a) (3-8b) (3-9a) (3-9b) (3-10a) (3-10b) (3-11a) (3-11b) (3-12a) (3-12b)

+H+=HSO-4 S024

(3-13a)

K = 10-1.91

(3-13b)

From the Eqs. (3-1) to (3-13), the Eh-pH diagram of sodium sulphide solution is constructed with element sulphur as metastable phase considering the presence of barrier (about 300 kJ/mol) or overpotential (about 3.114 mV) of sulphide oxidation to sulphate and shown in Fig. 3.7. It is obvious that the lower limit of potential of sodium sulphide-induced collectorless flotation of pyrite, pyrrhotite and arsenopyrite at various pH agree well with the potential defined respectively by reactions of Eq. (3-9) producing elemental sulphur. The initial potential 58

3.2 Nature of Hydrophobic Entity 0.8 0.6 0.4

C5 u: :r:

~

::t ir:

-;: ~ -0.2 H2S

-0.4

• Pyrite pH= 8 • Pyrite pH=11 o PyrrhotitepH =8

-0.6 -0.8

0

2

4

6

8

10

12

14

pH

Figure 3.7 Electrochemical phase diagram for sodium sulphide with elemental sulphur as metastable phase. Equilibrium lines (solid lines) correspond to dissolved species at 10-3 moIIL. Plotted points show the upper and lower limit potential of sulphur-included flotation of pyrite, pyrrhotite and arsenopyrite from literature

where these minerals begin to possess sodium sulphide-induced floatability is corresponding to the metastable elemental sulphur, indicating that elemental sulphur is hydrophobic entity. When above the upper limit potential, hydrophilic species such as sulphate is dominant, and the flotation ceases. The voltammetric behavior of pyrite at pH= 8.8 (see Fig. 3.8) shows that an anodic current commenced at about -0.25 V to give an anodic peak at about 0 V. On the reverse scan a cathodic current that appeared at the same potential could be presumed to represent the reduction of the initial oxidation products. According to the reaction (3-9), the formation of sulphur would be expected to occur at -0.26 V for the HS- concentration of 10-2 mol/L at pH= 8.8 which is consistent with an anodic current that begins to occur. Heyes and Trahar (1984) further compared the floatability of pyrite with the electrochemical and contact angle result reported by Walker and Richardson. Their results are listed in Fig. 3.9. It indicates that the onset of anodic current during a 80r--------------.,

o~

60 40

.§ §

20

~<

0

~

e

~ 7 -20

HS:O.Olmol/L

8 c- -40

-60 -80 '-----I-_.....J----..J~--'--_....I...----'-_--' -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 Potential(V, SHE)

Figure 3.8

Cyclic voltammogram for pyrite in buffer solution at pH = 8.8

59

Chapter 3

Collectorless Flotation in the Presence of Sodium Sulphide 200 160

80

~ 120 (1)

60

:<E

~

t::::s u u

:.a0

40

80

~

;> 0

o

~

o

-0.5 -0.3

-0.1 0 0.1

0.3

'-' (1)

(1)

20

40

0

'fOb~ Q)

l::

<e:

~

t) ~

c0

u

0

0.5

Pulp potential (V, SHE)

Figure 3.9 Anodic current and contact angle as a function ofpotential for sulphidized pyrite compared with flotation recovery (Heyes and Trahar, 1984)

potential sweep of a pyrite electrode and an increase in the contact angle with potential of sulphidized pyrite is in reasonable agreement with flotation edge. They also attributed the results to the production of sulphur in terms of reaction (3-9) by which the oxidation of hydrosulphide ion to sulphur should be expected at -0.213 V for the hydrosulphide ion concentration of 10-3 mol/L at pH = 8.

3.3 Surface Analysis and Sulphur-Extract The X-ray diffraction diagrams of pyrite before and after treatment with sodium sulphide are shown in Fig. 3.10, which appears to confirm the presence of sulphur because the diffraction band of elemental sulphur occurs after pyrite treated with

Na2S. (a)

800

0

No Na2S

400

'00

c (1)

.5 c 0

'B t'j

~

0

0 2000

(b)

1500

S

Na2S: 10-3. 1 mol/L

1000 500 10

20

30

40 50 60 Diffraction angle(0)

70

80

Figure 3.10 X-ray diffraction diagrams of pyrite (a) untreated; (b) treated by 8.56 x 10- 4 mol/L Na2S (Wang et aI., 1991a,b,c,d)

60

3.3 SurfaceAnalysis and Sulphur-Extract

Sun et al. (1993a) reported the effects HS- ion concentration on the adsorption of HS-, the amount of extracted sulphur and sulphur-induced flotation of pyrite as shown in Fig. 3.11. The results show that during sodium sulphide-induced collectorless flotation, it involves the adsorption of HS- ion on the mineral and the HS- adsorbed can be oxidized into sulphur to render pyrite and arsenopyrite surface hydrophobic due to the fact that the adsorption density of -HS- ion increases with the HS- ion concentration and the amount of extracted sulphur and hence the flotation rate increases with the increase of adsorption density. It suggests that the mechanism of sodium sulphide-induced collectorless flotation of pyrite takes place by reactions: (3-14) SCad) + H+ + 2e-

HScad) =

0.10

2.5

~

]

,-o

~

Rate constant

0.08 ~

.S

Adsorption 0.06

1.5

I

Extracted sulphur 1.0

0.04

o .~ 0.5

0.02

~

0.10 en

2.0

rJJ

:r:

(3-15)

l:::

oen

"'0

<

~_--'--_-J...-_--L.._ _L - - _ . . . 1 - - _ - - - I

o

5

10 15 20 25 HS- concentration (10-4 mol/L)

30

0.00

0.00

Figure 3.11 Effects of HS- concentration on the adsorption, the amount of neutral sulphurextracted frompyrite surface and collectorless flotation of pyriteat pH = 11.0 (Sun et aI., 1993a)

McCarron Jet al. (1990) used the X-ray photoelectron spectroscopy to analyze chalcopyrite and pyrite surface after being conditioned in sodium sulphide solutions. They found that multilayer quantities of elemental sulphur were produced at the surface of both minerals in 3 x 10- 3 and 3 x 10- 4 mollL sulphide solutions although for a given sulphide concentration, the surface coverage of elemental sulphur for pyrite was greater than that for chalcopyrite under open circuit conditions. Eliseev et al. (1982) concluded that elemental sulphur was responsible for the hydrophobicity of pyrite and chalcopyrite treated with sodium sulphide. Luttrell and Yoon (1984a, b) observed a shoulder due to elemental sulfur near 164 eV in the S (2p) spectra from relatively pure chalcopyrite floated after being conditioned at different pulp potential established by different hydrosulphide concentration. 61

Chapter 3

Collectorless Flotation in the Presence of Sodium Sulphide

3.4 Comparison between Self-Induced and Sodium Sulphide-Induced Collectorless Flotation It may be seen from the results in Chapter 2 that the floatability descends in the order of chalcopyrite galena, pyrrhotite, bornite, arsenopyrite and pyrite in self-induced collectorless flotation but the order is just reversed in sodium sulphide-induced flotation, the phenomena of which may be explained in the light of their rest potential as shown in Table 3.1. Table 3.1 Rest potential for sulphide minerals in water at pH = 4 (Hayes et al., 1987) and the order of their self-induced and sulphur-induced collectorless floatability Rest potential (V, SHE)

Self-induced floatability

Sulphur-induced floatability

Molybdenite

0.11

High

Low

Stibnite

0.12

Argentite

0.28

Galena

0.40

I I I

Bornite

0.42

Covellite

0.45

I I

I I I

Sphalerite

0.46

I

Chalcopyrite*

0.56*

I

Marcasite

0.63

~

~

Low

High

Minerals

Arsenopyrite Pyrite

* anomalous.

62

0.66

I I I I

Chapter 4 Collector Flotation of Sulphide Minerals

Abstract In the beginning, the mixed potential model, which is generally used to explain the adsorption of collectors on the sulphide minerals, is illustrated. And the collector flotation of several kinds of minerals such as copper sulphide minerals, lead sulphide minerals, zinc sulphide minerals and iron sulphide minerals is discussed in the aspect of pulp potential and the natureof hydrophobic entityis concluded fromthe dependence of flotation on pulp potential. In the following section, the electrochemical phase diagrams for butyl xanthate/water system and chalcocite/oxygen/xanthate system are all demonstrated from which some useful information about the hydrophobic species are obtained. And some instrumental methods including UV analysis, FTIR analysis and XPS analysis can also be used to investigated sulphide mineral-thio-collector sytem. And some examples about that are listed in the last part of this chapter. , Keywords collector flotation; electrochemical phase diagram; UV; FTIR; XPS Ever since the mixedpotential model has been proposed, the interaction mechanism between thio-collector and sulphide minerals has been usually explained on the basis of this model. The principle of the mixed potential model can be Xads X2 MX2 . F·ig. 4.. 1 Here E02 . Iy . 11y shown In rev' Erev , Erev' Erev respective schematica represent the reversible potential of reactions (1-1), (1-2), (1-3), (1-4), E:estpotential represents the rest potential of a mineral. Figure4.1 demonstrates that the reduction of oxygen is the only cathodic process. When E~~s < E:estpotential < E::2 , the interaction of thio collector with a mineral is dominant by electrochemical adsorption in Eq. (1-2). When E::2 < E:estpotential < E~~, the electrochemical reaction ofthio collector on a mineral produces collector-metal salts in Eq. (1-4). When E~~ < E:estpotential < E~~ , the thio collector is oxidized to its dithiolate like Eq. (1-3). Table 4.1 shows the measured rest potential of sulphide electrode in thio collector solutions at pH = 6.86 and the equilibrium potential calculated for possible processes. In terms of the mixed potential model, the reaction products should be metal collector salts between four thio collectors and galena and jamesonite; and should be disulphide between four thio collectors and pyrite and

Chapter 4

Collector Flotation of Sulphide Mineralse

arsenopyrite. For chalcopyrite the reaction products may be dixanthogen in xanthate solution and metal collector compound in other collector solution.

M S

M ~-----------

S Xads

s Erestpotential

E rev

M

S

(a) Adsorption mechanism of single collector molecule

::: e-(t--__

M S

E~2

M

- - - - - - - - - - - - E:estpotential

S

M

Xads E rev

S

(b) Adsorption mechanism of collector salt

E~ ~(

M S

s ~ - - - - - - - - - - - Erestpotential

E~~

M S

M Xads Erev

S

(c) Adsorption mechanism of di-collector

Figure 4.1 The mixed potential model of redox reaction on sulphide surface Table4.1 The measured rest potential in 10- 4 mol/L thio collector solution at pH = 6.86 (Feng, 1989; Yu et aI., 2004a,b,c,d,e,f,g; Zhang et al., 2004a,b,c,d,e,f)

PbS

CuFeS2

FeS2

FeAsS

PbSbS

Reversible potential (V, SHE)

Ethyl xanthate

0.08

0.19

0.295

0.26

0.15

0.176

Butyl xanthate

0.04

0.11

0.30

0.245

0.09

0.107

Dithiocarbamate

0.09

0.145

0.32

0.28

0.13

0.168

Dithiophosphate

0.13

0.20

0.48

0.37

0.21

0.340

Rest potential (V, SHE)

Collectors

64

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

Although there have been a lot of investigations on the interactions of sulphide minerals with thio-collectors in terms of the mixed potential principle, there are still much controversy about the products formed on a sulphide mineral in the presence of a collector -in different conditions. In the following sections, the effects of potential on the flotation and formation of surface products of many kinds of sulphide minerals will be introduced based on the results of flotation, electrochemical measurement, surface analyses and thermodynamic calculations.

4.1 Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity 4.1.1 Copper Sulphide Minerals 1. Chalcocite The collector flotation of chalcocite has been studied in some detail (Basiollio et aI., 1985; Heyes and Trahar, 1979; O'Dell et aI., 1984; Richardson et aI., 1984, 1985; Walker et al., 1984). Figure 4.2 compares the results reported by different authors. It can be seen that the lower potential limit of flotation found in the different experimental conditions agrees favorably and is independent of pH; but the upper limit is pH dependent. 100 80 ,,-....

'$.

'6

60

OJ ;>

0

o

OJ

40

~

20 0'----_......0....--_----'---_----1._ _'""'--.......&..-----'------' -0.5 -0.3 -0.1 0.1 0.3 0.5

Potential(V, SHE)

Figure 4.2 Flotation recovery of chalcocite with ethyl xanthate as a function of potential: curve L-from Basiollio et al. (1985), solution pH= 11 (la),8(lb),5(lc); curve 2-from Heyes and Trahar (1979), solution pH = 11(2a) and 8(2b); curve 3-from Richardson et al. (1984), at pH = 9.2

Gaudin and Schuhmann (1936) and Harris and Finkelstein (1977) have shown that the most likely adsorbed hydrophobic entity is cuprous xanthate which could be formed by a combination of the oxidation of chalcocite and the reaction of the

65

Chapter 4

Collector Flotation of Sulphide Mineralse

oxidation product (cupric-ion) with xanthate (Repel and Pomianowski, 1977). It was assumed therefore that the reactions to account for the formation of cuprous xanthate may be: Cu 2S + EX- =

CuS + CuEX + e"

(4-1)

According to the date in literature (Du Tietz, 1975) CuEX=Cu+ + EX-

(4~2a)

K sp= [Cu+][EX-] = 10- 19.28

(4-2b)

The change of standard free energy of reaction (4-2) is

~d' = - RTlnKsp= - 2.303RTIgKsp = 110.009(kJ/mol)

(4-3a)

Since (4-3b) Thus

=~G~u+ - ~Go' = 50.208 -110.009 = -59.801(kJ/mol) ~G~uEX - ~G~x-

(4-3c)

The change of standard free energy of reaction (4-1) is

~GO = ~G~uEX + ~G~us - ~G~x- - ~G~U2S = -59.801- 48.·953 + 86.190 = -22.564(kJ/mol)

(4-4)

EO = b.Go = -22.564 x1000 =-0.234(V) nF 2x96490

(4-5)

Eh = - 0.234 - 0.059Ig[EX-]

(4-6)

Similarly, Cu 2S + 2EX- =

2CuEX + S + ze

EO= -0. 173(V) {

Eh = - 0.173 - 0.0591g[EX-]

EO = 0.225(V) {

66

E; = 0.225 - 0.01481g[X-] + 0.0073751g[SO~-] - 0.059pH

(4-7a) (4-7b)

(4-8b)

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

(4-9a)

EO== 0.147(V) {

(4-9b)

E; = 0.147 - 0.0295Ig[X-] + 0.007375Ig[S20~-] - 0.04425pH 100 ".-...,

450

80

225 ~ E

~

'6 Q)

;> 0

60

M

I

Q) l-

c0

.~ 0

G:

0

.......

U

,.....

40

-0.6

C Q) t: ::s

-225 u

20

o

0

<;»

-0.4

-0.2

0

-450 0.2

Potential (V, SHE)

Figure 4.3 Linear voltammogram and floatability of a bed of chalcocite particles in aqueous solution of ethyl xanthate (1: 4.7 x 10- 5 mol/L, from Heyes and Trahar, 1979; 2: 1.44 x 10- 5 mol/L, from Richardson et al., 1984; 3 is a cyclic voltammogram: 5 mV/s, 1.9 x 10- 5 mol/L KEX ,buffered at pH = 9.2 in 0.05 mol/L borate, from O'Dell et al., 1986)

However, the calculated potential for CuEX formation corresponding to a xanthate concentration of 4.7 x 10- 5 mol/L are, respectively, 0.021V for Eq. (4-6), 0.082V for Eq. (4-7b) and -0.391 V for Eq. (4-8b) (for pH= 11 and a sulphate ion 4 concentration of 10- mol/L), which do not agree with the observed lower limiting flotation potential of Heyes and Trahar (1979) in Fig. 4.2 (curve 2). The calculated potential for reactions (4-1) also do not correspond with the results of curve 1 and 3 in Fig. 4.2. Potential sweep measurements on a chalcocite particle bed electrodes in solutions containing ethyl xanthate (O'Dell et al., 1986)were found to display the same features as on a single particle electrode of natural chalcopyrite showing three anodic which were first reported by Kowal and Pomianowski (1973). The voltammogram and flotation results are compared in Fig. 4.3. It can be seen that an anodic current occurs at the same potential as the flotation edge. It is generally accepted that the first anodic peak in this system arises from the chemisorption of xanthate on the mineral surface. The second anodic peak is due to the formation of bulk copper ethyl xanthate. Therefore if the anodic current and the flotation edge correspond to the formation of CUX, there is a substantial difference between the observed and calculated potential. However, if the reaction is assumed to be Eq. (4-9), the calculated potential for CuX formation are -O.IIV at pH= 8 and -0.24V at pH= 11, which are in reasonable agreement with the lower limiting flotation potential indicating that a reaction of the form of reaction (4-9a) may be the reaction ofCuX formation. 67

Chapter 4

Collector Flotation of Sulphide Mineralse

Figure 4.3 also shows that there is an upper flotation edge and that its potential varies with the changes of pH. Heyes and Trahar (1984) attributed the cessation of flotation to the decomposition by the reactions of the type: (4-10a)

EO = 1.402(V) 2CuEX+4H 20=2Cu(OH)2 + (EX)2 +4H++4e-

(4-10b)

EO = 0.891(V) The calculated decompositionpotential are 0.783 Vat pH= 5, +0.516 V at pH=8, and +0.251 V at pH = 11 for [HCuO;] = 10--6 mol/L for reaction (4-1Oa). Neither fits the data in Fig. 4.2. The calculated decomposition potential however are 0.59 V at pH= 5,0.411 V at pH= 8 and 0.23 V at pH= 11 for reaction (4-10b), which are in excellent agreement with Basilio's results in Fig. 4.2, although the results of Heyes and Trahar in Fig. 4.2 show that there is no upper limiting potential for chalcocite at pH = 8. Thus the fall in recovery will commence at the potential predicted by the reaction of the type (4-10b). The reaction (4-10) also gives an evidence that dixanthogen dose not confer floatability on chalcocite in the region of potential in Fig. 4.2. 2. Chalcopyrite The influence of pulp potential on the floatability of chalcopyrite is shown in Fig. 4.4 for an initial concentration of 2 x 10-5 mol/L ethyl xanthate and butyl xanthate. The lower flotation potential is -O.IV for KBX and OV for KEX. The hydrophobic entity is usually assumed to be dixanthogen (Allison et al., 1972; Woods, 1991; Wang et al., 1992) by the reaction (1-3). The calculated potential in terms of reaction (1-3), are, however, 0.217 V and 0.177 ~ respectively, for ethyl and butyl xanthate oxidation to dixanthogen for a concentration of 2 x 10-5 mol/L, which corresponds to the region of maximum recovery but not to the lower limiting potential for flotation, indicating that some other surface hydrophobicity to the mineral. Richardson and Walker (1985) considered that ethyl xanthate flotation of chalcopyrite may be induced by the reaction: CuFeS2 + X- =

CuX + FeS2 + e

(4-11)

EO = -0.096(V) For [EX-] = 2 x 10- 5 mol/L, CuX would be produced at 0.18V. Again, it does not correspond with the observed lower limiting flotation potential. Voltammograms obtained with chalcopyrite electrode (see Fig. 4.5) show that the anodic current appears at the potential corresponding to the formation of dixanthogen by the reaction of the form (1-3). Therefore, it is reasonable to assume

68

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity ]00 80 ~ 60

'6 Q)

~

o Q)

40

.......- pH=10, Wang et aI., ]992

0:::

20

--6-

pH=9, Feng, ]990

~

pH=9.2, Richardson and Walker, ]985

o'-------I~---L------l...-----L..-----J.----L----l -0.3 -0.]

0.] 0.3 0.5 0.7 Potential (V, SHE)

0.9

Figure 4.4 Flotation recovery of chalcopyrite as a function of pulp potential

that dixanthogen may be responsible for the hydrophobicity of chalcopyrite in xanthate-induced flotation. Recently, Cheng and Iwasaki (1992) reported that the flotation of chalcopyrite commenced only at oxidizing potential (above O.4V) with sodium isopropyl xanthate as collector at near neutral pH under nitrogen aeration, which is obviously attributed to dixanthogen. 800 600

- - -Blank ]0-3 mol/L BX --- ]0-2 mol/L BX

~

Eo

400

5,

200

=<

C

·00

c

~

0

E ~;:i -200

u

Scan rate 40mVIs pH=9.]8 buffer solution 0.1 mol/L KN0 3

-400 -600 -400

-200

0

200

400

E(mV, SHE)

Figure 4.5 Voltammograms for chalcopyrite electrode in borate solution at pH = 9.2

4.1.2 Lead Sulphide Minerals 1. Galena There is lack of consensus in the published electrochemical studies of flotation of galena and the nature of the important hydrophobic species responsible for

69

Chapter 4 Collector Flotation of Sulphide Mineralse

flotation. The limiting potential of flotation has different values for different pretreatment procedures. Many authors (Allison and Finkelstein 1971; Finkelstein et aI., 1975; Poling, 1976; Finkelstein and Poling, 1977) claimed that lead xanthate was formed predominantly, dixanthogen could not be formed in the xanthateoxygen-galena systems, and that lead xanthate was extremely hydrophobic. Other researchers (Tolun and Kitchener, 1963 -1964; Toperi and Tolun, 1969; Woods, 1971; Gardner and Woods, 1977; Ahmed, 1978), however, found that dixanthogen was a product of xanthate oxidation at galena and the presence of dixanthogen was essential for the efficient flotation of galena with xanthate collectors, which led these researchers to the conclusion that the interaction mechanism of galena with xanthate was the coexistence of lead xanthate and dixanthogen. In the case hydrophobic entity is assumed to be lead xanthate, lead xanthate would be formed by the reaction of the form (1-4) including: (4-12a) (4-12b) PbS+2X- =PbX 2 +S+2e-

(4-12c)

According to the date in literature (Du Tietz, 1975; Garrels and Christ, 1965; Vanghan and Craig, 1978) PbX 2=Pb2+ + 2XKsp,EX= [Pb2+][EX-]2= 10- 16 .7 , ~G ~G

0'

0"

Ksp,BX= [Pb2+][BX-]2= 10- 18 (4-13)

= - RTlnKsp,Ex = - 2.303RTIgKsp,Ex = 95.288(kJ/mol)

(4-14a)

=-RTlnKsp,Bx=-2.303RTIgKsp,Bx= 102.705(kJ/mol)

(4-14b)

Since (4-14c) Thus

~G~bEX2 - 2~G~x- = ~G~b2+ - ~Go' = -24.309 - 95.288 = -1 19.597(kJ/mol)

(4-14d)

~G~bBX2 - 2~G~x- = ~G~b2+ - sa" = -24.309 -102.705 = -127.0 14(kJ/mol)

(4-14e)

The change of standard free energy of reaction (4-12a) is (4-15a)

70

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

For ethyl xanthate

I1GO = 179.852(kJ/mol), EO = 0.233(V)

(4-15b)

I1GO = 172.435(kJ/mol), EO = 0.223(V)

(4-15c)

For butyl xanthate

Similarly, the change of standard free energy of reaction (4-12b) is For ethyl xanthate

I1GO = 125.525(kJ / mol), EO = 0.163(V)

(4-16a)

For butyl xanthate

I1GO = 110.691(kJ/mol), EO = 0.143(V)

(4-16b)

The change of standard free energy of reaction (4-12c) is For ethyl xanthate ~GO

-26.921(kJ/mol), EO = -0. 14(V)

(4-17a)

I1GO = 34.334(kJ/mol), EO = -0. 178(V)

(4-17b)

=

For butyl xanthate

The xanthate-induced flotation of galena with an initial xanthate concentration of 10-4 mollL is presented in Fig. 4.6 and Fig. 4.7. It can be seen from Fig. 4.6 that flotation ranges from 0 V to + 0.41 V, the floatability of galena is bad when the potential is lower than -50 mY, while potential ranges from -50 mV to 0 m~ the recovery increase rapidly. The potential upper limit of flotation is about 410 mV (Sun et al., 2002). Figure 4.7 further shows that the potential range of galena flotation is different at different conditions such as pH and reagent concentration. At a given pH, the flotation potential range is wider at higher concentration of xanthate and at given concentration, the flotation potential range is wider in weak acidic pH media. Figure 4.8 shows the voltammograms of galena electrode in the presence of xanthate at natural pH value. It shows that the lower limiting potential for the flotation of galena is corresponding to the initial oxidation potential, which would be formed by the reaction of the form (4-12). When the concentration of butyl xanthate is 10- 4 mol/L, the calculated equilibrium potential for reactions (4-12a), (4-12b) and (4-12c) with the concentration of all dissolved species of 6 10- mollL are respectively -63mV, -84mV, -58m~ at pH= 6.8, which are quite close to the lower edge of flotation, indicating that the anode oxidation of galena attribute to the reaction (4-12). 71

Chapter 4

Collector Flotation of Sulphide Mineralse

80 70

•

60 ""'

'
'6

50

o

40

11)

> 0

11)

~

30 20

I0 '--'---'L..-~'--",--",--",--",--",---'---'--.J-.l....L......J -200 -100 0 100 200 300 400 E(mV,SHE)

Figure 4.6 Relation between recovery of galena and pulp potential in the presence of condition (BX: 10-4 mollL, KN03: 0.1 mollL; flotation time: 2 min, at natural pH)

-4 Ig c

-5 2

4

6

8

10

12

pH

Figure 4.7 The Eh-pH-c three dimensional diagram for flotation of galena in the presence ofxanthate (1- floatability area; 2-non floatability area)

The upper limit for the flotation of galena attributed to the decomposition by the reaction of the type: (4-18)

EO = 1.225(V) (4-19)

EO 72

=a.8(V)

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity 0.0003 0.0002 0.0001

~

e

~

0.0000 I I I I I

-0.0001

E} :

-0.0002 -0.0003

E2

I I I

-0.4

-0.2

0.0

0.2

0.4

0.6

E(V, SHE)

Figure 4.8 Voltammograms for galena electrode in the presence of xanthate at natural pH (BX: 10- 4 moIlL, KN0 3 : 0.1 mol/L, scan rate: 0.5 mV/s)

The calculated equilibrium potential for reactions (4-18) and (4-19) with the concentration of dissolved species 10-6 mollL are respectively 417 mV and 409 m~ When the potential is higher than 400 mV, the recovery of galena fall gradually due to the reaction (4-18) and (4-19). Trahar et al. (Trahar, 1984; Guy and Trahar, 1984) presented convincing evidence that galena floats rapidly under the conditions in which lead xanthate is present on the mineral surface, but not under the conditions in which dixanthogen should be present on the mineral surface when galena is ground in sodium sulphide solution. They reported that the xanthate-induced flotation of galena at pH= 8 and 11 with an initial xanthate concentration of 2.3 x 10- 5 mol/L for grinding in sodium sulphide extended from 0 V to +0.3 V at pH = 8 and from ~.1 V to +0.1 V at pH = 11. When galena is ground in an oxidizing environment (ceramic mill), flotation occurred at a much wider and lower potential, the flotation was rapid over a wide potential range extending from -0.35 to +0.35 V at pH= 9.2 for a butyl xanthate concentration of 2 x 10- 5 mollL and -0.5 to 0.35 V at pH = 8 for a ethyl xanthate concentration of 2.3 x 10- 5 mollL. Evidently, the potential of formation of lead xanthate according to reaction (4-12c) is independent of pH, which is not consistent with the flotation results. Whereas, the calculated equilibrium potential for reactions (4-12a) and (4-12b) in 2.3 x 10- 5 mol/L ethyl xanthate and 10- 6 mol/L sulphate or thiosulphate are respectively -0.97mV, -98 mV at pH= 8 and -229 mv, -231 mV at pH= 11, indicatingthat reaction (4-12b) seems to fit their flotation data, i.e. the formation of lead xanthate and the presence of thiosulphate due to the oxidation reaction. Electrochemical investigations on the galena/oxygen/diethyl dithiocarbamate (DDTC) system have also been conducted and indicated that lead diethyl dithiocarbamate (PbD2) was formed which conferred the mineral hydrophobic (Yarar et al., 1969; Wang et al., 1992) .Wang found that the interaction ofDDTC with galena is characterized by fast rate. The PbD2 can be firmly adsorbed on 73

Chapter 4

Collector Flotation of Sulphide Mineralse

galena. DDTC also has good characteristics in strong alkaline media for galena flotation, Gu Guohua (1998) studied the electrochemical behavior of galena using DDTC as a collector in strong alkaline media. The results show that, in strong alkaline media (pH> 12.5) the potential of electrochemical adsorption of DDTC on galena is in the range of 0 to 0.2 V. The voltammogram of galena electrode in the presence ofDDTC in strong alkaline media is shown in Fig. 4.9. It is evident that the anodic current peak arises at about 0 to 0.2 V in different scan potential ranges. This indicates that there exists an anodic reaction: (4-20)

.t> =- 0.301(V) The reversible potential of reaction(4-20) corresponding to DDTC concentration of 4xl0- 5mol!L is calculated to be -0.044 V which is quite close to the initial oxidation potential in the voltammogram. The anodic peak of the formation of PbD 2 almost coincides with the peak of galena self-oxidation (Gu and Wang, 2000). In this case, because the products of the oxidation of galena itself are HPbO; and S, there exists a vis-reaction of the formation ofPbD2 : (4-21) During the cathodic scan, the first cathodic peak is corresponding to the formation of PbS again, the other is the reduction of surface surplus sulfur.

pH=12.8 PbS-PbD2,S DDTC: 4 x 10-5 mol/L

HS-S PbS-PbDbS

I

PbS-PbD2,S lm/vcm?

+

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E(V, SHE)

Figure 4.9 Cyclic voltammograms of galena electrode at potential scan of 20 mV/s (Background solution: pH = 12.8 buffer solution plus 0.5 mol/L KN0 3 at 25°C, 4 X 10- 5 mol/LDDTC)

74

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

Whenthe upperlimitof the potential scan is above 0.4~ the anodic currentpeak of formation of PbD2 occurs due to two reactions. One is that of reaction (4-20). The other reaction of the formation ofPbD 2 may be: (4-22)

EJ = 0.082(V) Figure 4.10 represents voltammograms at different DDTC concentration at pH = 12.8. The upper limit of potential scan is 0.2 V and 0.5 V respectively. The anodic currentpeak of the formation of'Pblr, increases with the increase ofDDTC concentration, meanwhile the anodic current peak of the oxidation of galena decreases.

DDTC(mol/L):

pH=12.8

1: 0

2: 4x 10-5 3: 4x 10-4 4: 4x 10-3

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E(V, SHE)

Figure 4.10 Cyclic voltammograms of galena electrode in different DDTC concentration at potential scan of 20 mV/s (Background solution: pH = 12.8 buffer solution plus 0.5 mol/L KN0 3 at 25°C)

It can be concluded that there exists a critical concentration of DDTC between o and 4xl0- 3 mol/L at given pH. Above this concentration, PbD2 is formed by anodic reaction (4-20). Below the criticalvalue, the oxidation of galena occurs as the major anodic process. It also follows from Fig. 4.10 that the degree of the cathodic current peak increased is lower than that of the anodic peak rising. This indicates that PbD2 is not reduced completely in the cathodicprocess. Figure 4.11 shows three cycles of the voltammograms of the galena electrode at pH= 12.8 with DDTC concentration 4xl0-3 mol/L, Because of the high DDTC concentration, the oxidation of galena itself is depressed. The anodic currentpeak of PbD2 arises evidently at 0 - O.2V. However, the anodic currents decrease in 75

Chapter 4

Collector Flotation of Sulphide Mineralse

pH=12.8 DDTC: 4x 10-3 rnol/L

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E(V, SHE)

Figure 4.11 First three consecutive cycling voltammograms of galena electrode at potential scan of 20 mV/s (Background solution: pH = 12.8 buff solutions plus 0.5 mol/L KN0 3 at 25°C, 4x10- 3 mol/L DDTC)

tum when the electrode is cyclically scanned within a range of -O.9V and O.6V

and the anodic peak potential tends to be negative slightly. This phenomenon may be attributed to the incomplete reduction or the faradic de-sorption of PbD2. The incomplete reduction of PbD2 at the galena surface in the cathodic scan may holdbackthe anodic process. The faradic de-sorption ofPbD2 in the strong reducing condition causesthe anodic currentto decrease. 2. Jamesonite The influence of pulp potentialon floatability ofjamesonite is shown in Fig. 4.12 for an initialconcentration of 1 x 10- 4 mol/Lethyl xanthate (EX), dithiocarbamate (DDTC) and ammonium dialkyl dithio-phosphate (ADDP). If the upper and lower 100...---------------, 80

20

...... EX

........ DDTC -+-ATTP

o~-'--~--'-----'---'---""&'-'---'----'------'--------' -200

0

200

Eh(mV, SHE)

400

600

Figure 4.12 Flotation recovery of jamesonite as a function of pulp potential in 4 the presence of collector (collector concentration: 10- mol/L, pH = 8.8)

76

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

potential limit for flotation was defined at 50% flotation recovery, it has been shown that jamesonite could be floated at a wide range of pulp potential at a given pH. The potential of flotation is in the range of 0.I2V to 0.47V for EX. Using DDTe as a collector the potential range of flotation is from O.27V to

O.57~

Jamesonite has a wider potential range of flotation with ADDP as a collector, the lower limit flotation potential is -0.I8V, the upper limit is 0.37Y. Figure 4.13 presents the lower (EhL) and upper (Ehu) limiting flotation potential 4 of jamesonite as a function of pH with collector concentration of 1 xl 0- mol/L. It can be seen that the lower (EhL) and upper (Ehu) limiting flotation potential is changed with the pH value. The flotation ofjamesonite may occur only at a range of pulp potential EhL < Eh < Ehu. The flotation potential with DDTC as a collector is higher than that with EX as a collector.

600

~ 400

::c r./)

';>

S

~200

o 4

-4-1:Eh (u) ----1: Eh (1) -""2:Eh (u) -.-2:Eh (1 )

6

8

pH

10

12

Figure 4.13 The lower (EhL) and upper (Ehu) limiting flotation potential of jamesonite as a function of pH (I-EX, 2-DDTC, collector concentration: 1 xIO- 4 mol/L)

Figure 4.14 is the Tafel curves of jamesonite under the conditions of different concentration of DDTC in natural pH solution. Obviously, the corrosive potential moves negatively and its corrosive current decreases with the DDTC concentration increasing. DDTC can obviously inhibit the anodic corrosion of jamesonite due to its chemisorption. In cathodic area, the Tafel slope in the presence ofDDTC is bigger than that in the absence of DDTC, and the cathodic curves under the conditions of different DDTC concentration are almost parallel and their Tafel slopes only change a little. These demonstrate that the chemisorption of DDTC on the surface of jamesonite electrode also inhibits the cathodic reaction, but the chemisorption amount of DDTC is a little and almost not affected by the DDTC concentration due to their negatively electric properties of DDTC anion and the electrode surface. This reveals that there is a little DDTC chemisorption on the mineral even if the potential is lower (i.e., negative potential). 77

Chapter 4

Collector Flotation of Sulphide Mineralse

400 , - - - - - - - - - - - - - - - - - - - - ,

300 200

>'8

~

pH=6.9 cone. of DOTe 1: 0.0 mol/L 2: 1.0x 10-3 mol/L 3: 5.0x 10-4 mol/L 4: 1.0x 10-5 mol/L

100

o -100 -9

-8

-7

-6

-5

-4

IgI

Figure 4.14 Tafel curves of jamesonite electrode in 0.1 mol/L KN0 3 solution under the conditions of different DDTC concentration (unit of I: A/cm2) 4

In anodic area, the anodic Tafel slope at 1.OxlO- mollL ofDDTC concentration is remarkably bigger than that in the absence of DDTC, so the adsorption of DDTC anion is quite intensive on jamesonite electrode, and further results in the passivation to inhibit the anodic reactions. But the anodic Tafel slope decreases when DDTC concentration is between 5.0 xl 0 -4 mol/L and 1.0 xl 0 -3 mollL. This results from the corrosive potential moving negatively to promote the anodic reaction. With the deposition of the sulfur and collector metal salts from the anodic reaction, corrosive currents reduce rapidly with the enhancement of electrode potential continuously. Consequentally, there must be an optimum concentration of DDTC between 1.0 x 10- 4 mol/L and 5.0 xl 0- 4 mol/L to inhibit more effectively anodic corrosion reaction. Figure 4.15 is CV diagrams ofjamesonite electrode in 0.1 mol/L KN0 3 buffer solution containing different concentration of DDTC. The collector-metal-salts from the electrochemical reactions of jamesonite with the DDTC solutions of different concentration can effectively adhere to the surface of the mineral electrode to inhibit anodic process so that their ap, peaks are nearly the same. But the inhibition actions for anode electrochemical reactions are different at ap2' apj , apa peaks when the DDTC concentration is different: (1) the current intensity of apj , ap, peaks increases with the enhancement of the DDTC concentration; (2) not only are the ap2peaks most obviously passive but also the 4mol/L peak current intensity of ap, and ap, (dot line) at 5.0xl0of the DDTC concentration is lower than that (dash line) at 1.OxlO- 3 mol/L of the DDTC concentration. Consequentially, there must be a critical concentration of DDTC. DDTC and its metal salts can inhibit the electrochemical reactions on the mineral in an extensive potential range when the DDTC concentration is lower than the critical value. But DDTC can promote the electrochemical reaction on the mineral when the DDTC concentration is higher than the critical value. It means 78

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

4.0

l' o

0.0

,.~

o ~ -4.0

1.0x 10-4 mol/L --- 1.0x 10- 3 mol/L ...... 5.0X 10-4 mol/L

-8.0 -1.0

-0.5

0.0

E(V)

0.5

1.0

Figure 4.15 Voltammograms for jamesonite electrode in 0.1 mollL KN0 3 buffer solution of pH = 6.86 containing different concentration of DDTC (scan rate: 50 mV/s)

that the collector-salts can not cover the surface of the mineral electrode in this case. DDTC can effectively inhibit the electrochemical reaction from 0 to 300 mV whatever the experimented concentration of DDTC is, but in the point of flotation practice, the lower the collector concentration, the lower the reagent cost. So, the optimum concentration of the collector must be some value in the range from 1.0xl0-4 to 5.0xl0-4 mol/L. This result is unanimous with the results discussed in Fig. 4.14. Figure 4.16 is the electrochemistry impedance spectrum (EIS) of jamesonite electrode under the conditions of different DDTC concentration, and its EIS parameters are shown in Table 4.2. It follows that: (1) The electrochemical resistance is smaller in the absence of DDTC, but increases four times in presence of DDTC. So the adsorption of DDTC on jamesonite results in reducing the reaction rate of the corrosive electrochemistry in open circuit potentia1. (2) The electrochemical impedance gradually decreases but does not vary very much with the DDTC concentration increasing. It indicates that DDTC takes part in the electrochemical reaction and the reaction rate increases as the DDTC concentration enhances. As contrasted with it, the passivation of the collectorsalts of reaction production on the mineral electrode further inhibits the anodic reaction so that the electrochemical resistance is wholly much bigger than that of self-corrosive reaction in the absence ofDDTC. (3) Capacitance impedance loop is smaller and its interfacial capacitance of mineral/solution is bigger in the absense ofDDTC. But the capacitance impedance loop obviously enlarges and the interfacial capacitance becomes small in the presence ofDDTC. With the DDTC concentration increasing, there is no obvious change of the capacitance impedance loop, but its interfacial capacitance increases. 79

Chapter 4

Collector Flotation of-Sulphide Mineralse

16000

12000

~ N""

••

8000

•

...

••

• •

4000

• •

•

•

• DOTe concentration (mol/L)

• 0.0 • 1.0X10-4 ·5.0x10-4 • 1.0x10-3

~

o

•

•

• • •

I

10000

20000 Zre(W)

30000

40000

Figure 4.16 EIS of jamesonite electrode in 0.1 mol/L KN0 3 solution containing different concentration of DDTC

Usually, as the organic matter with small electrical medium constant can push aside H20 molecule with big electrical medium constant, e becomes small and Cd rises according to the following formula: (4-23) Table 4.2 EIS parameters of jamesonite electrode under the conditions of different conc.ofDDTC Direct current volt, Vd (V)

0 5.0 xl0- 4

1.0 xl0- 3

45.60

42.79

39.95

107.6

154.3

186.5

0

1.0 xl0-

Electrochemical resistance, R, (ill)

11.80

(IlF/cm 2)

274.4

Cone. ofDDTC (mol/L) Interfacial capacitance, Cd

4

Here, e is the electrical medium constant of the adsorbed matter, and d is the diameter of the adsorbed molecule. Consequentally, the interfacial capacitance in the presence of DDTC is smaller than that in absence of DDTC, so mineral hydrophobicity will increase when an organic matter is adsorbing on a mineral electrode. It reasonably explains the experimental appearance in Fig. 4.12. The interfacial capacitance increases with the DDTC concentration added. The relationship among potential difference f//l of diffusion layer, the electric charge density q on the surface of an electrode and the concentration c of a solution according to Gouy, Chapman and Stem model theory is as follows. q=

80

F ) ~8cRT808r sinh(IZIV!l 2RT

(4-24)

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

As DDTC adsorbs on jamesonite electrode chemically, the double electric charge layer is treated as a plate capacitor, the capacitance C t of the tight layer as a constant, and the change of the capacitance of the double electric charge layer is designated to the capacitance Cz of the diffusion layer. Thereby, the tight layer and the diffusion layer are looked upon as two series capacitances according to the method from Cooper and Harrison, then: (4-25) Here, ({Ja is the potential of the whole double electric charge layer. When the electric charge density q on the surface of an electrode and the concentration c of a solution are very small, namely, the absolute value of f/1tF is small far from RT, the formula can be spread out in terms of a progression, and its high grade terms can be ignored, then: _

({Ja -'II,

+_1 ~2C808r F

C,

RT

(4-26)

'II,

c is very small in a dilute solution; the second term can be ignored because it is much smaller than the first term in the formula above, then, ({Ja'Zf/1t. This indicates that the capacitance of the diffusion layer is just the capacitance of the whole double electric charge layer. Because (4-27) The formulas (4-26) and (4-27) are transferred into: Cd

~C2 =.!L= C,«({Ja -'II,) =~2C808r F ~

~

~T

(4-28)

The formula of a plate capacitor is as follows:

C d -

GoGr

I

(4-29)

Compare the formulas (4-28) and (4-29), then

I =J.-~RT808r F 2c

(4-30)

Therefore, the effective thickness I is in inverse proportion to cl/Z the square root of adsorbed ion concentration. It reasonably explains the appearance that the capacitance increases with the DDTC concentration added and also indicates that 81

Chapter 4

Collector Flotation of Sulphide Mineralse

the adsorption behavior of DDTC on the surface of jamesonite is similar to ion characteristic adsorption. The adsorption ofDDTC is very tight onjamesonite. DDTC is an organic anion with comparatively big G and variable shape. After adsorption and bonding with mineral electrode, it can be effectively close to the electrode surface so that I becomes small and G becomes big. So its capacitance increases with the DDTC concentration added, amount of adsorption increasing. Therefore, the corrosive potential of jamesonite moves negatively with the DDTC concentration added. It promotes the anodic reaction of jamesonite. On the contrary, DDTC metal salt of the reaction production covered tightly the electrode surface inhibits its anodic reaction to result in the decrease of the corrosive current. There must be an optimum concentration of collector DDTC between 1.0xl0- 4-5.0xl0- 4 mol/L.

4.1.3

Zinc Sulphide Minerals

1. Sphalerite Only limited studies on the electrochemical behavior of sphalerite have been reported, perhaps due to its high electrical resistivity. The Relation between recovery of sphalerite and pulp potential is presented in Fig. 4.17 with an initial 4 butyl xanthate concentration of 10- mol/L. It can be seen from Fig. 4.17 that flotation begins at 0 V, the upper limit potential is 0.31 V. 70 60 ~

50

~;>-

40

?J(

0

u

(!)

0:::

30 20 10

0

100

200

300

400

Eh(mV, SHE)

Figure 4.17

Relationbetween recovery of sphaleriteand pwp potential in the presence ofBX (BX: 10-4 mol/L, KN0 3 : 0.1 mol/L, flotation time: 2 min, at natural pH)

Figure 4.18 shows the voltammograms of sphalerite electrode in the presence of xanthate at natural pH. The initial oxidation potential is -10mV and the peak potential of further oxidation is 300 mV. For collector flotation of sphalerite, the 82

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

initial flotation potential of sphalerite may be corresponding to the oxidation potential. The reaction is as following: ZnS + 2X- ---» znX 2 + S + 2e-

(4-31)

EJ =-0.05(V) (4-32)

EJ = 0.256(V) The potential is 200 mV based on reaction (4-31) and 10mV on reaction (4-32) at natural pH. It can be inferred that the initial reaction of sphalerite corresponds to reaction (4-32). For the collector flotation of sphalerite, the upper limit potential of flotation may be corresponding to the following decomposition reactions: (4-33)

EJ = O.956(V) (4-34)

EJ = 0.743(V) Assuming the concentration of all dissolved species to be 10- 6mollL, the reaction potential are 0.37 V and 0.32 V respectively, corresponding to reactions (4-33) and (4-34) and the oxidation peak potential in Fig. 4.18. Therefore the upper limit potential of flotation of sphalerite may depend on reaction (4-33) or (4-34), i.e. the decomposition of zinc xanthate and the formation of zinc hydroxide or oxy-zinc species. 0.0018 0.0009 0.0000

<' -0.0009 E

~ -0.0018 -0.0027 -0.0036 -0.4 -0.2

0.0 0.2 0.4 Eh(mV, SHE)

0.6

0.8

Figure 4.18 Cyclic voltammetric curves for sphalerite electrode at natural pH in the presence of condition (BX: 10- 4 mol/L, KN0 3 : 0.1 mol/L, scan rate: 0.5 mV/s)

83

Chapter 4

Collector Flotation of Sulphide Mineralsc

Figure 4.19 shows the relationship of Eh-pH-c of sphalerite in the presence of xanthate. The floatable region of sphalerite is dependent on the pH, potential and the concentration of reagent addition. It can be seen that there exists an appropriate potential range for zincsulphide flotation for a givenpH andreagent concentration. Within this range, minerals represent good floatability. Above this range, minerals floatability descends, and the limiting potential of flotation always corresponds to specified reactions. 0.8 2

W :c C/l

»

lir

0.4 0.2 0

-0.2 -3 Ig c

-4

-5 2

4

6

pH

8

12

10

Figure 4.19 The relationship of Eh-pH-c of sphalerite in the presence ofxanthate I-floatability area; 2-non floatability area

2. Marmatite

The influence of pulp potential on the flotation of marmatite at different pH is given in Fig. 4.20usingethyl xanthate as a collector. In acidic pH media, marmatite exhibits a wide floatable potential range and the upper potential limit of flotation 100,--------------------, 80 ~

(:J!.

~> o

~

60

-+- pH=4.7

- - pH=8.8 - . - pH=12.1

40

20~ o

100

200

300

400

500

600

700

800

Eh(mV, SHE)

Figure 4.20 Flotation recovery of marmatite as a function of pulp potential with ethyl xanthate as a collector at different pH (KEX: \0 -4 mollL)

84

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

of marmatite is 620 mV. In alkaline pH media, the floatability of marmatite is very poor. The initial potential of ethyl xanthate flotation of marmatite as low as oV suggests the formation of zinc xanthate according to similar reaction (4-32) to sphalerite because the formation potential of ethyl dixanthogen is 0.17 V for 4 concentration 10- mollL based on the following reaction. 2EX-

~ (EX)2 +

2e-

Eo = -0.06(V) {E

h

= -0.06 - 0.059Ig[EX-]

(4-35a) (4-35b)

However, the decomposition potential of zinc xanthate into dixanthogen is above 0.3 V according to reactions (4-33) or (4-34) and the upper potential limit of flotation of marmatite extends to 620 mV, which indicates the coexistence of dixanthogen on marmatite in this condition. The difference of flotation behavior and hydrophobic entity between sphalerite and marmatite may be due to the existence of iron in marmatite. Figure 4.21 reveals the influence of pulp potential on the flotation ofmarmatite at different pH using dithiocarbamate as a collector. Figure 4.21 shows that marmatite has good floatability with a maximum recovery of about 90% in acidic pH media with a potential range 300 -750 mV when DDTC is used as a collector. In alkaline pH media, the floatability of marmatite becomes very poor in various potential regions. If the collector metal salt was formed into marmatite like sphalerite, the reaction between mannatite and dithiocarbamate may be written simply as follows: ZnS+2D- ~ZnD2 +S+2e-

(4-36a)

EJ =-0.21(V) (4-36b)

EJ = -0.128(V) 4

The calculated potential are, respectively 26 mV and -260 mV for 10- mollL DDTC at pH = 4.7, which are not consistent with the flotation beginning potential in Fig. 4.21. On the other hand, the reaction of dithiocarbamate oxidation to thiouram disulphide can be written as 2DDTC -=(DDTC)2 + 2e-

(4-37a)

Eo = -0.015(V) { E; = -0.015 - 0.059Ig[DDTC]

(4-37b)

4

The calculated potential is 221 mV for 10- mollL DDTC, which reasonably corresponds to the initial potential of marmatite flotation in Fig. 4.21. Therefore, 85

Chapter 4

Collector Flotation of Sulphide Mineralse

the hydrophobic entity of dithiocarbamate on marmatite may be assumed to be disulphide. 100

80

~

'$. 60

'6 Q)

> o o

~

40 -+-pH=4.7 ____ pH=8.8 ---+- pH=12.l

o

100 200

300 400

500

600

700 800

900

Eh(mV, SHE)

Figure 4.21 Flotation recovery of marmatite as a function of pulp potential with dithiocarbamate as a collector at different pH (DDTC: 10- 4 mol/L)

The voltammograms for marmatite electrode in different pH buffer solutions are presented in Fig. 4.22. It can be seen from Fig. 4.22(a) that the first anodic peak occurred at about 200 mV, which may be due to the oxidation of dithiocarbamate to disulphide. The anodic oxidation peaks at higher potential may be attributed to the further oxidation of marmatite to form oxy-sulphur and zinc/iron hydroxide resulting in flotation descending. Figure 4.22(b) shows that no anodic oxidation peaks occurred at lower potential region in alkaline media for marmatite electrode in xanthate and dithiocarbamate solution indicates no formation of collector species on marmatite. When the potential increases to higher potential region, the occurrence of anodic oxidation peak may be due to the self-oxidation of marmatite. It accounts for no flotation of marmatite in alkaline media either using xanthate or dithiocarbamate as collectors.

4.1.4 Iron Sulphide Minerals 1. Pyrite The influence of potential on the floatability of pyrite with butyl xanthate as a collector has been determined and the result is given in Fig. 4.23. It follows that 4 flotation begins at 0.1 V for an initial KBX concentration of 10- mollL. The flotation potential ranges from 0.10 V to, +0.31 V.

86

4.1

Pulp Potential Dependence of CollectorFlotationand Hydrophobic Entity 40

20

o -20 ___

1 l

-40

-60 -80

~ -100

pH=4.0

-120

Solid line: without DDTC Dash line: with DDTC

-140 -160 -180

L..-.-...I----J.----I..----L-----L..-..L....---L...----L.----I---L-...J.-....L...-....L...-..L..-.l---L--...J

-0.6 -0.4 -0.2

0.0

0.2

E(V)

0.4

0.6

0.8

1.0

(a)

150 100 50

---E

.-.?~~~~)

--......----._._._.-.,:;../

('l

u

~

::i ~

0

pH=9.18 Solid line: DDTC Dot line: EX Dash line: collectorless

-50 -100 -0.4 -0.2

0.0

0.2

0.4

E(V)

0.6

0.8

1.0

1.2

(b)

Figure 4.22 Voltammograms for mannatite electrode in different pH buffersolution (a) in 0.1 mol/L Na2S04 media at scanning rate of 50mV/s; (b) in 0.1 mol/L KN0 3 media at scanningrate of20 mV/s 80 70

---~

'6

60 50

Q)

;;> 0

o

~

40 30

E2

20 10

100

200 300 Eh(mV, SHE)

400

Figure 4.23 Recovery of pyrite as a function of pulp potential in the presence of condition (BX: 10- 4 mol/L,KN0 3 : 0.1 mol/L, flotation time: 2 min) 87

Chapter 4

Collector Flotation of Sulphide Mineralse

It is generally assumed that the product of the pyrite-xanthate interaction in air-saturated solution is dixanthogen (Majima and Takeda, 1968; Ball and Rickard, 1976; Gardner and Woods, 1977; Janetski et al., 1977) generated by the reaction (4-35). The observed lower flotation potential is consistent with the calculated reversible potential of about 0.1 V for the oxidation of xanthate to dixanthogen, taking If to be -0.128 V for butyl xanthate/dixanthogen. Although Trahar (1984) found that the optimal flotation of pyrite occurred in the potential region where dixanthogen was formed, he also observed that flotation began at -0.15 V much lower than the reversible value for the xanthate/dixanthogen couple, which led him to suggest that dixanthogen could not be the sole hydrophobic entity responsible for the flotation of pyrite. The formation of an iron xanthate species may be possible in rendering pyrite floatable. Figure 4.24 shows the voltammograms of pyrite electrode in the presence of xanthate at natural pH. Figure 4.23 and Fig. 4.24 indicate that the initial flotation potential is around 106 mV, which is corresponding to initial potential of anode oxidation. When xanthate concentration is 10- 4 mol/L, the potential is about 100 mY. The flotation upper limit edge of pyrite is 300 mY, which is near to the upper limit edge of collectorless flotation. It is assumed that hydroxide may be responsible for the hydrophilicity of pyrite. 3 2

<'E

':t

b

0

'-'

........

-1 -2

-3

-0.4

-0.2

0.0 0.2 Eh(mV, SHE)

0.4

0.6

Figure 4.24 Cyclic voltammetric curves for pyrite electrode at natural pH in the presence of condition (BX: 10- 4 mol/L, KN0 3 : 0.1 mol/L, scan rate: 0.5 mV/s)

Figure 4.25 further shows the relationship of Eh-pH-c of pyrite in the presence of xanthate. It can be seen that the range of flotation potential becomes narrow with the increase of pH and the decrease of concentration. Under the condition of alkaline media, floatability of pyrite is very poor in various concentration conditions and potential range.

2. Pyrrhotite The relationship between pyrrhotite flotation recovery and pulp potential is presented in Fig. 4.26. It can be shown that the flotation of pyrrhotite has different

88

4.1

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity

0.6

0.5 0.4 W 0.3

:c

"",---J-~~-+-rl

L~~~+--t--t-T 2 _-J:~~-t-ll

0.2

[/J

">

i:;J 0. 1

o -0 .1 L--~-

- 3.0 - 3.5 - 4.0 Igc -4.5

10 ~:S~~~Y---]01 12 '--8 6 4 -5 .0 2 pH

Figure 4.25 The relationship of Eh-pH-c of pyrite in the presence ofxanthate (1: floatability area; 2: non floatability area) 100r-----------------,

~

"
'6

60

\l)

>

0

u \l) 0::

40

20

EX ---DDTe --ADDP

ojL.O"""O--'----'2-'0"""0----'----:-30.LO,----'--4---'O'-:-O---'------:s:-'-OO".------l...---:-60-'-:O-L..-...::-70-'O Eh(mV, SHE)

Figure 4.26 Relationship between recovery of pyrrhotite and pulp potential in the presence of collector (collector concentration: 10- 4 mol/L, pH = 8.8)

suitable pulp potential for different collectors. If the flotation potential upperand lower limits were defmed according to a flotation recovery of 50% respectively (R = 50%), the relationship of pyrrhotite flotation upper (Ebu) and lower (EbL) limit potential is presented in Fig. 4.27. It can be seen that the flotation of pyrrhotite may occurs only in some rang of the pulp potential at various pH. At pH =8.8, the optimal potential range for the flotation of pyrrhotite is about 150 - 320 mV using ethyl xanthate as a collector and the maximum flotation occurred at potential 250 mV. The optimal potential range for the flotation of pyrrhotite is about 280 - 550 mV using dithiocarbamate as a collector and the maximum flotation

89

Chapter 4

Collector Flotation of Sulphide Mineralse

occurred at potential 300 mV. The optimal potential range for the flotation of pyrrhotite is about 200- 400 mV using ammonium dialkyl dithio-phosphate as a collector and the maximum flotation occurred at potential 300 mV. Pyrrhotite has a wider range of potential for flotation at pH = 4 - 12 using dithiocarbamate than using ethyl xanthate as a collector.

600

~ w ::r: ir: >"

400

S

~

200

---l:Eh (u) --.- 1:Eh(l)

..-...- 2: Eh(u) -.-2:Eh (l) 6

8

pH

10

12

The lower (EhL) and upper (Ehu) limiting flotation potential of pyrrhotite as a function of pH (1: EX, 2: DDTC, collector concentration: 10- 4 mol/L)

Figure 4.27

The anodic scan section of cyclic voltammetry for pyrrhotite electrode, respectively, in pH= 7, 8.8, 11, 12.1, 12.7 buffer solution with dithiocarbamate are presented in Fig. 4.28. The cyclic voltammograms curve at pH = 8.8 is also presented in Fig. 4.28. It can be seen that the anodic current peak emerges at about 0.1V. As pH increases, the peak moves to the left. This peak may correspond to the formation of disulphide. When the concentration of dithiocarbamate is 10-4 mol/L, the oxidation of dithiocarbamate forming disulphide is Eh = O.22V, which agrees with the results in Fig. 4.26. When the hydrophobic entity disulphide was formed, the flotation of pyrrhotite could be possible. It can be also seen from Fig. 4.28 that the place of anodic peaks.moves towards the left with the increase of pH. When pH= 12.7, the anodic peak disappears, and the anodic currentincreases rapidly. It indicates that only the oxidation of pyrrhotite takes place on the surface of pyrrhotite. In addition, no disulphide was formed at pH = 12.7. So the flotation of pyrrhotite could not be carried out. 3. Arsenopyrite Arsenopyrite exhibits similar xanthate induced floatability to pyrite. The result obtained by Feng (1989) is presented in Fig. 4.29. It follows that flotation occurs at potential where dixanthogen is formed, confirming the mechanism of formation of dixanthogen from Allison et al. (1972). 90

4.2

Eh-pH Diagrams for the Collector/Water/Mineral System

pH=7.0

pH=8.8

I100JlA

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

E(V)

0

0.1

0.2 0.3

Figure 4.28 The anodic scan section of cyclic voltammetry for pyrrhotite electrode in different pH buffer solution at potential scan of 10mV Is (Background solution: buffer solution plus 0.1 mollL KN0 3, DDTC: 10- 4mollL) 80 60 ,-,

'$-

'6 Q)

;> 0

Arsenopyrite KBX: 10-4·7 mol/L pH=9

40

o

Q)

~

20 0L..--_ _ -0.5 -0.3

.L..--_ _.L..::.-_ _.L..--_ _.L...--.....3IL------l

-0.1 0.1 Potential(V, SHE)

0.3

0.5

Figure 4.29

Flotation recovery of arsenopyrite as a function of pulp potential (Arrow indicates the reversible potential of butyl xanthate/dixanthogen couple)

4.2 Eh-pH Diagrams for the CoUector/Water/Mineral System Electrochemical phase diagrams have been used to investigate the collector water mineral system in which the experimental potential for flotation is compared with thermodynamic equilibriums for reactions in mineral/oxygen/collector system to 91

Chapter 4

Collector Flotation of Sulphide Mineralse

obtain the useful information about the hydrophobic species and Eh-pH area of formation of the hydrophobic entityin orderto predictflotation behaviors (Pritzker and Yoon, 1984a,b; Toperi and Tolun, 1969; Hepel and Pomianowski, 1977; Abramov et al., 1977; Chander and Fuerstenau, 1975a,b, 1979; Woods et al., 1990; Wang and Forssberg, 1991; Forssberg et aI., 1984; Wang and Hu, 1989).' Abramov et aI. (1977) draws an electrochemical phase diagram for the chalcocitelbutyl xanthate/oxygen system and the observed rest potential which give firm bubble attachment, assumes total dissolved species concentration to be 10-6 mollL and total carbonate concentration to be 10- 4 mollL with butyl xanthate concentration of 10- 4 mollL and shows that the experimental attachment of bubbles to the chalcocite surface is observed when the potential correspond to the field of stable coexistence of chemisorbed collector, cuprous xanthate and indicates that dixanthogen should not be the hydrophobic species.

4.2.1

Butyl XanthatelWater System

The electrochemical reaction leading to the productionof dixanthogen is 2BX- =

(BX)2 + 2e-

(4-38a)

t> =-O.l(V) { Eh = -0.1 - 0.059 Ig[BX-] 2HBX=

(BX)2 + 2H+ + 2e-

(4-39a)

EO= 0.201(V) {

(4-38b)

(4-39b)

Eh = 0.201- 0.059 Ig[HBX--] - 0.059pH

(4-40a) Ka=7.9x10- 6

(4-40b)

The upper Eh limit for water stability is definedby the reaction: 1

H20=-02 +2H+ +2e

_

2

Eo= 1.23(V) {

Eh = 1.22- 0.059pH

(4-41 a)

(4-41b)

The lower Eh limit of water stability is definedby the reaction: H2=2H+ +2e92

(4-42a)

4.2 Eh-pH Diagrams for the Collector/Water/Mineral System

E h=-0.059pH

(4-42b)

Thus, the various equilibrium lines, together with the experimental potential at which flotation recovery is greater than 50%, are shown in Fig. 4.30 and Fig. 4.31. It can be seen from Fig. 4.30 that the Eh-pH area for the flotation of galena should be in the same area as the formation of lead butyl xanthate, whereas the lower limiting potential of chalcopyrite are corresponding to that of the formation of cuprous xanthate but the floatability is extended to wider Eh range where dixanthogen is dominant. Figure 4.31 shows that the Eh-pH region of the flotation of pyrite and arsenopyrite occurs only in the area where dixanthogen is found. 0.8

(BXh

0.6 0.4

HBX

0.2

.

-0.6 -0.8

..

Galena ~.~ - - - Chalcopynte .~

-0.4

o

2

4

6

8

<, ~ 12 14

10

pH

Figure 4.30 Electrochemical phase diagram for the butyl xanthate/oxygen system and the observed lower (EL) and upper (£'1) limiting flotation potential of galenaand chalcopyrite at which flotation recoveryis greaterthan 50% (BX-: 2 xlO- 5 mol/L) 0.8 r--------~----____, 0.6 0.4

GJ ~

-;;

0.2

0 BX: 10- 4.7 mol/L - - - Arsenopyrite "-, . . . -Pyrite

~-0.2 -0.4

"

<,

-0.6 -0.8

<,

"-,,"-

L---.J-_--J....-_...I--~_

o

2

4

6

pH

8

<,

'

..... ___I..__~~

10

12

14

Figure 4.31 Electrochemical phase diagram for the butyl xanthate/oxygen system and the observed lower (EL ) and upper (ED) limiting flotation potential of pyrite and arsenopyrite at which flotation recoveryis greaterthan 50% (BX-: 2 xlO- 5 mol/L) 93

Chapter 4

4.2.2

Collector Flotation of Sulphide Mineralse

Chalcocite-Oxygen-Xanthate System

The electrochemical reaction of formation of cuprous xanthate is defined by the following equation (4-43a)

{ E; = EJ - 0.04425pH + 0.007375Ig[ S20~- ] - 0.0295 Ig[X-] E~X = 0.147(V)

(4-43b)

The decomposition of cuprous xanthate by oxidation is defined by the following reactions: 2CuX + CO~- + 2H 20 =

Cu2(OH)2C03 + X 2 + 2H+ + 4e-

E~X = O.53(V)

{ Eh= EJ - 0.0295pH - 0.01475 19[CO~-] 2CuX + 4H 20 =

{

2Cu(OH)2 + X 2 + 4H+ + 4e-

E~X = 0.89(V)

(4-44b)

(4-45a) (4-45b)

E h = EJ - 0.059pH

2CuX +4H 20 =2CuO~- + X 2 +8H+ +4e-

{

(4-44a)

(4-46a)

E~X =1.8(V) E h = EJ + 0.0295 19[CuO;-] - 0.118pH

(4-46b)

The stability of cuprous xanthate in high pH media is determined by the reaction of the type: CuX + 40H- =

{

CuO~- + X-

+ 2H 20

(4-47a)

6

K~X = 1.45 x 10-

Ig[OH-] = (1/ 4)(lg[X-][CuO~-] -lgK)

(4-47b)

The dissolution of copper hydroxide at high pH is CU(OH)2 =

CuO;- + 2H+

(4-48a)

K = 1.57 X 10-31 CU2(OH)2C03 + 20H- =

K = 1.57 X 103 94

2Cu(OH)2 + CO~

(4-48b)

4.3 SurfaceAnalysis

The oxidation of xanthate ion into dixanthogen is defined by the Eq. (4-35), in which E~X/(EX)2 = -0.06 . In drawing the equilibrium lines, the concentration of dissolved species is assumed to be 10- 6 mollL. The electrochemical phase diagrams for the chalcocite/ xanthate/oxygen system are shown in Fig. 4.32 with an ethyl xanthate concentration of 4.7 xl0- 5 mollL. Figure 4.32 shows that the lower and upper limiting potential pH area of flotation of chalcocite at which the recovery is greater than 50% (Basiollio et al., 1985; Heyes and Trahar, 1977) is located in the region where cuprous ethyl xanthate is stable species, indicating that the hydrophobic entity is cuprous ethyl xanthate. The flotation ceases at higher potential by the oxidation into CU2(OH)2, CuO~- and dixanthogen, and at high pH conditions by the decomposition into CuO~- , S20~- . 0.8

-0 H;o- __2___

0.6 CU2(OH)2C03 0.4

W

J:

r./1 >~

(EX)2

0.2

0

CuEX, EX-

~-0.2 -0.4

S20 jCuO~-

CU2S

-0.6

--- __H 20 H2 - - - -0.8 L..--_i---_i--------I-----.;~'---~'---.---l 13 7 8 10 11 12 9 pH _

_____J

14

Figure 4.32 Electrochemical phasediagram for the chalcocite/ethyl xanthate/oxygen system and the observed lower (EL) and upper (ED) limiting flotation potential at which flotation recovery is greater than50%, dashed lineis theformation of dixanthogen

4.3

Surface Analysis

Some instrumental methods have been used for the investigation of sulphide mineral-thio-collector system such as infra-red (IR) spectroscopy (Mielezarski and Yoon, 1989; Leppinen, 1990; Persson et al., 1991; Laajalehto et al., 1993) and X-ray photoelectron spectroscopy (XPS) (Pillai et al., 1983; Page and Hazell, 1989; Granoet al., 1990; Laajalehto et al., 1991). These surface sensitive spectroscopic techniques can be applied for the direct determination of the surface composition at the conditions related to flotation.

95

Chapter 4

Collector Flotation of Sulphide Mineralse

4.3.1 UV Analysis of Collector Adsorption on Sulphide Minerals 1. Adsorption of Dithiocarbamate on Jamesonite In order to characterize the adsorption species on mineral surface, DDTC is oxidized into the dimmer by adding definite H20 2 into the DDTC solution, which then is extracted by cyclohexane to determine its UV spectrum. As seen from the UV spectrum in Fig. 4.33, there are three UV absorbance peaks at 230 nm, 261 nm, 280 nm respectively. The maximum absorbance peak is at 230 nm, the next peak is at 260 nm, and the weak peak is at 280 nm. The peak at 230 nm can serve as a characteristic absorbance peak, and the peak at 260nm results from absorbance overlapping of diethyl dithiocarbamate and its dimmer. The UV spectra of the cyclohexane solution extracted from jamesonite acted by DDTC solution, shown in Fig. 4.34, indicates that the adsorption of diethyl dithiocarbamate on the surface of jamesonite is almost the same at pH = 4 and 7, but decreases with the increasing of pH in the base solution. Because the UV spectra in Fig. 4.33 and Fig. 4.34 are similar with peaks at 230 nm and 260 nm, it indicates that the hydrophobic species on jamesonite should be the mixture of diethyl dithiocarbamate and its dimmer. 2. Adsorption of Dithiocarbamate and Xanthate on Marmatite Figure 4.35 presents the UV spectra of the cyclohexane solution extracted from marmatite after acted by DDTC. It can be seen that there exist three UV absorption peaks at 230 nm, 260 nm and 280 nm. The hydrophobic entity may be the mixture of diethyl dithiocarbamate and its dimmer on marmatite. 70 60 50

--e 40 Q)

o

c ro

.-e0

30 rn ~ Q) > 20 .~

Q) ~

10 0 -1 0 L...-..l.-~..L---I--..L.-....I---I--L---I.-.L..-...L...~..L---I--..L.---1..---I 200 220 240 260 280 300 320 340 360

A(nm)

Figure 4.33 UV spectrum of the cyclohexane solution extracted from the oxidized DDTC solution by H202

96

4.3

70

Surface Analysis

2

l:pH=4.0 2:pH=7.0 3:pH=9.3 4:Ca(OHh

60 ~

~ 50 (1)

o

§ 40

~ 0

rrJ

30

~ (1) >

20

.~

a) ~

10 0 -1 0 ..............--'-----'--..L--......_......--'--___'__~'___'___..____'__~I......_....I..____'

200

220 240 260

280 300 320 340 360 A(nm)

Figure 4.34 UV spectra of the cyclohexane solution extracted from jamesonite acted by DDTC solution 100 l:pH=4.0 2:pH=7.0 3:pH=9.3 4:Ca(OHh

80 ~

~

Q) o

§

.D

60

~

0

r/)

.D

cj Q)

>

.~

40

a) ~

20

0

4

200

220

240

260

280 A(nm)

300

320

340

360

Figure 4.35 UV spectra of the cyclohexane solution extracted from marmatite acted by DDTC solution

Figure 4.36 shows the UV absorbance spectra of diethyl dixanthogen (EX)2, diethyl monothiocarbonate (MTC-) and diethyl thiocarbonate (EPX). The UV adsorption peaks of (EX)2 lie in about 238 nm (strong) and 286 nm (weak). The ratio of adsorption intensity of two peaks is 2. UV peak of MTC lies in about 225 nm (strong), EPX lies in about 348 nm (strong) and 221 nm (weak). After ethyl xanthate interacts with marmatite, the UV spectra of the hydrophobic species on marmatite surface extracted by cyclohexane are shown in Fig. 4.37. There are three UV peaks, lying in 228 nm, 263 nm and 286 nm respectively. It is

97

Chapter 4 Collector Flotation of Sulphide Mineralse

very similar to Fig. 4.36. Same results were reported by Fomasiero (1995). Therefore, the UV peak at 228 nm is an overlapped peak by these peaks of238 nm (EX)2, 225 nm (MTC) and 221 nm (EPX). The UV peak at 263nm results from the overlapping of the mediate peaks of (EX)2 and EPX. UV peak at 286nm is just the peak of EX2 in Fig. 4.36. It may be derived that after ethyl xanthate interacts with marmatite, the main adsorption product on its surface may be dixanthogen with a little of EPX- and MTC- metal salt. 18 2

16

.r\

14

I .\

/

,

4\

12

/,

.0 'S:

.~ 10 o ~ 8

I:X 2 2:X3:EPX4:MTC-

\

I

.

" \; , ,.... 1

1'\

.~,

\

,:' ".

,/

,/

I

j'\.. . . . ,/ . .

'\

3

\,1

\,\J

~ ~

6:' :

4

\\....

'\\,'.".

2

\~'"

o200 220

\ ....:

j

.....,.

.~.; :,: \ ...... \

'

240 260 280 300 320 340 360 380 400 A(nm)

Figure 4.36 Molar absorptive spectra of ethyl xanthate and its derivatives 70..--------------------. 60

~ 50

l:pH=4.0 2:pH=7.0 3:pH=9.3 4:Ca(OHh

Q)

C)

I:

]

$-

o

40

V':J

.D

~;> 30

.~

~

20 10 0L---L---L...-....I....--J.--l----I..-L.--J....--L...---L..-..1--1-........L....-.L.--:c:

-L--J

180 200 220 240 260 280 300 320 340 360 A(nm)

Figure 4.37 UV spectra of the cyclohexane solution extracted from marmatite acted by EX solution

98

4.3

Surface Analysis

From the flotation results in Fig. 4.21 and the voltammogram in Fig. 4.22, it is derived that the hydrophobic entity of collector on marmatite is mainly of disulphide of xanthate and dithiocarbamate, which is further confirmed by the above UV analysis. However, the UV analysis also suggested the coexistence of collector salts. We propose that the initial oxidation products of xanthate and dithiocarbamate on marmatite should be mainly of disulphide. At higher potential, the adsorbed disulphide may be decomposed and react with surface zinc species to form some parts of collector salts as in the following reactions: (4-49a)

EJ = 0.8337(V)

{ E = 0.8337 h

0.059pH = 0.2921(V)

Zn(OH)2 + (EX)2 + 2H++ 2e-=Zn(EX)2 + 2H20

EJ = 0.5563(V)

{E

(4-49b) (4-50a) (4-50b)

h = 0.5563 - 0.059pH = 0.01468(V)

4.3.2

FTIR Analysis of Adsorption of Thio-Collectors on Sulphide Minerals

Persson et al. (1991) used diffuse reflection infrared Fourier transform (DRIFT) spectroscopy to study the interactions between galena, pyrite sphalerite and ethyl xanthate. They provided the evidence that the DRIFT spectrum of oxidized galena treated with an aqueous solution of potassium ethyl xanthate is practically identical with that of solid lead (II) ethyl xanthate, which can be formed as the only detectable surface species on oxidized galena. Dialkyl dixanthogen is formed as the only surface species in the reaction between oxidized pyrite and aqueous solution of potassium alkyl xanthate. Leppinen (1990) used FTIR-ATR techniques to study the adsorption of ethyl xanthate on pyrite, pyrrhotite and chalcopyrite. He found that the FTIR signals of the xanthate species on pyrite occurred at approximately the same position as those of bulk dixanthogen together with a surface product, the signal of which coincided well with that of ferric ethyl xanthate. The adsorption product of KEX on pyrrhotite was much the same as that on pyrite but no signal of iron xanthate species was observed on the surface of pyrrhotite. On chalcopyrite the signals of diethyl dixanthogen and a metal xanthate compound closely resembling copper (I) xanthate were observed. These results led Leppinen (1990) to conclude that the main adsorption product of ethyl xanthate on pyrite and pyrrhotite is diethyl dixanthogen. An iron xanthate surface compound is also observed on pyrite at monolayer coverage. A mixture of copper xanthate compound and diethyl

99

Chapter 4 Collector Flotation of Sulphide Mineralse

dixanthogen occurs on chalcopyrite. Leppinen et al. (1989) also studied the adsorption of ethyl xanthate on chalcocite and galena and examined the influence of pulp potential. He reported that on chalcopyrite the xanthate adsorption at pH = 9.2 initially resulted in the formation of dixanthogen at potential above the reversible potential for the xanthate/dixanthogen couples. Copper ethyl xanthogen is at high potential.

1. Adsorption of Ethyl Xanthate on Pyrrhotite The FTIR reflection spectra of ethyl xanthate, and iron ethyl xanthate were,reported to show the following characteristic absorption bands of ethyl xanthate: the stretching vibration bands of the C-O-C at 1100 - 1172 em -1 and C=S at 1049 em -1 and 1008 em -1. When dixanthogen was formed, the stretching vibration bands changed. The characteristic absorption bands are 1260 cm- 1, 1240 cm- 1, 1020 cm- 1 and 1105 em -1. When iron ethyl xanthate was formed, the stretching vibration band ofC=S shifted to 1029 cm- 1 and 1005 cm- 1 (Mielezarski, 1997; Leppinen, 1990). FTIR reflection spectra of ethyl xanthate adsorption on pyrrhotite are shown in Fig. 4.38. It can be seen that the characteristic absorption bands of diethyl dixanthogen which at 1023 cm- 1, 1242 cm- 1, and 1263 cm- 1 appear on the surface of pyrrhotite, indicate that the dominating hydrophobic species on pyrrhotite surface is dixanthogen. Further, the effect of pulp potential on the adsorption of ethyl xanthate on pyrrhotite was determined and the results are presented in Fig. 4.39. It follows that at pH = 8.8 the ethyl xanthate adsorption on pyrrhotite is mainly of dixanthogen independent of potential in the range of 140 - 250 mV due to the occurrence of almost the same dixanthogen characteristic band. However, the intensity of the IR signals changes at various potential. It demonstrates the intensity of the IR signals of the characteristic dixanthogen peaks and hence

1500

1000 Wave numbers (crrr")

Figure 4.38 FTIR reflection spectra of ethyl xanthate adsorption on pyrrhotite (pH = 7.0, KEX: 5 x 10- 3 mol/L; s, = 297 mY)

100

4.3

Surface Analysis

-~ ~~ 'D 'o::t ~~

~

~

0

~

Q)

o

s::

S o

190mV

Q)

r;: Q)

~

~

0; 0

160mV

0\ 00

00

~

\0

~

rrl

r-

0 ~

~

r-

0

140mV

00

~

rrl

\0

0

~

\0

~

rr-

00 oo

rrl

s:

1500

1000 Wave numbers (crrrl)

Figure 4.39 FTIR reflection spectra of ethyl xanthate adsorption on pyrrhotite at different pulp potential (pH = 8.8, KEX: 5 x 10- 3 mol/L)

dixanthogen adsorption on pyrrhotite decreases with the decreasing potential from 250 to 140 mV At pH= 8.8, flotation recovery and dixanthogen adsorption on pyrrhotite decrease with the decrease of pulp potential from 250 to 140 mV as shown in Fig. 4.26. The effect of pH on the FTIR signal intensity of xanthate adsorption on pyrrhotite and pyrrhotite flotation recovery is plotted in Fig. 4.40. The signal intensity has been measured based on the characteristic absorption 100 80

0 .;n

.-....

~

'6

t::

60

Q)

,5

Q)

;:> 0

u

Q)

"

Cd

t::

40

bJJ

'00 --0--

20 0

~

~

Recovery

~

-+- FTIR signal intensity

2

4

6

8

10

12

14

pH

Figure 4.40 The effect of pH on FTIR signal intensity of pyrrhotite adsorption xanthate and on pyrrhotite flotation recovery

101

Chapter 4

Collector Flotation of Sulphide Mineralse

peak of diethyl dixanthogen at 1020 em -1. It can be seen from Fig. 4.40 that the IR signal intensity corresponds well to flotation recovery. In weak acidic pulp solution, the IR signal intensity is the strongest and the flotation recovery of pyrrhotite is the maximum.

2. Adsorption of Ethyl Xanthate on Marmatite Figure 4.41 presents the FTIR reflection spectra of ethyl xanthate adsorption on marmatite at different pH. The characteristic absorption peaks 1210, 1108, 1025 cm -1 occurred. It has been reported that the characteristic absorption bands of dixanthogen are 1260, 1240, 1020 and 1105 cm- 1 and those bands of zinc xanthate are: 1030, 1125 and 1212 cm- 1 (Mielezarski, 1986; Leppinen, 1990). It is derived from Fig.4. 41 that there may exist the mixture of dixanthogen and zinc xanthate because both distinct peaks of dixanthogen and xanthate salt appeares in Fig. 4.41, which further confirms the results from the UV analysis in Fig. 4.36 and Fig. 4.37. It can also be seen from Fig. 4.41 that the intensity of the IR peaks is strong indicating the stronger adsorption of xanthate on marmatite accounting for its good floatability in weak pH media. When pH increased above 7, only a very weak peak appeared in the spectra indicating very weak or no adsorption of xanthate on marmatite accounting for its very poor floatability in alkaline pH media.

2

1200

1000

800

Wave numbers(crrr ')

Figure 4.41 FTIR reflection spectra of ethyl xanthate adsorption on marmatite (1: pH= 7.0; 2: pH= 4.7; 3: pH= 2.2; KEX: 5 ><10-3 mol/L) 102

4.3 Surface Analysis

3. Adsorption of Ethyl Xanthate on Jamesonite It was reported that when lead ethyl xanthate was formed, the stretching vibration band of C=S shifted to 1018 and 996 cm -1. The stretching vibration band of

C-O-C shifted to 1112 and 1207 cm- I (Leppinen, 1990).

The FTIR reflection spectra of ethyl xanthate adsorption on jamesonite are shown in Fig. 4.42. It can be seen that the characteristic absorption bands of lead ethyl xanthate at 1020, 1112 and 1206 cm- I appeared on the surface ofjamesonite, indicating the primary hydrophobic species onjamesonite surface to be lead ethyl xanthate. It is possible that antimony ethyl xanthate was formed on jamesonite surface simultaneity like lead ethyl xanthate. 78~----------------'----'

77 76 ~ 75 ~ 74 o ~

g

~

73 72

~ 71

70

69 68

L . . - - _ - - - J . - . . _ - - - I ._ _- - L . . - _ - - - L_ _

1500 1400

1300

1200

1100

~_

1000

____'__

900

_______I

800

Wave numbers (crrr ')

Figure 4.42 FTIR reflection spectra of jamesonite adsorption ethyl xanthate (pH = 7.0; KEX: 5 x 10- 3 mollL; E h = 307 mY)

The effect of pulp potential on the adsorption of ethyl xanthate on jamesonite was further examined and the results are presented in Fig. 4.43. At pH= 4.7 the ethyl xanthate adsorption on jamesonite was mainly of metal ethyl xanthate independent of potential in the range of280- 385 mV due to the occurrence of the same metal ethyl xanthate characteristic band. However, the intensity of the IR signals was changed as the pulp potential varied. The intensity of the IR signals of the characteristic reaches peaks of metal ethyl xanthate and hence the metal ethyl xanthate adsorption on jamesonite decreases with the descending of the pulp potential from 385 - 280 mV. At pH = 4.7, the flotation recovery and metal ethyl xanthate adsorption on jamesonite decrease with the decrease of pulp potential from 385 to 280 mV as shown in Fig. 4.12. The effect of pH on FTIR signal intensity of xanthate adsorption on jamesonite, which was measured near 1206 em-I, and on flotation recovery is plotted in Fig. 4.44. It can be seen that the IR signal intensity corresponds reasonably well to the flotation recovery. In acidic pH media, the IR signal intensity was the strongest and jamesonite showed best flotation. The IR signal intensity gradually decreases and the flotation of jamesonite exhibits good flotation in weak alkaline pH media and decreases in strong alkaline pH media. 103

Chapter 4

Collector Flotation of Sulphide.Mineralse

\0

--

V)

r-'

-

0 N

280mV

("'f")

N

-

~ 0

.-. .-.

~

1500 1400

\.0

r-.

00 0

\0

V)

00 0

1300 1200 1100 1000 Wave numbers(crrr")

900

800

Figure 4.43 FTIR reflection spectra of ethyl xanthate adsorption on jamesonite at different pulp potential (pll = 4.7, KEX: 5 x 10- 3 mol/L) 100. . . - - - - - - - - - - - - - - - - - - . 1 0 90

,,-....

80 70

8

'-' '*' 60 50 >

S o

g 40 ~ 30 20

-0-

Recovery

-e- FTIR signal intensity

10

o

' - - _ . . L . - _ . . . . L - - _ - - ' - - _ - - ' - - _ - - - L - _ - - - ' - - _.......

2

4

6

pH

8

10

12

14

0

Figure 4.44 The effect of pH on FTIR signal intensity of xanthate adsorption on jamesonite and jamesonite flotation recovery

4. Adsorption of Dithiocarbamate on Pyrrhotite The FTIR reflection spectra of DDTC (solid) and FeD3 are shown in Fig. 4.45. Only at the 700 - 2000 em -1 region most of the important functional groups vibrations of diethyl dithiocarbamate are observed. From Fig. 4.44, it can be seen 104

4.3

Surface Analysis

that many reflection spectra peaks appeared. When C=S was connected to atom N, there were a lot of stronger reflection spectra appearing in a range .of 15701395 cm", 1420-1260 cm- I and 1140- 940 em-I. The characteristic absorption bands of diethyl dithiocarbamate (DDTC) are 1475, 1455, 1412, 1375, 1260, 1202 and 984 em-I. The characteristic absorption bands of iron diethyl dithiocarbamate (FeD 3) are: 1491, 1454, 1354, 1270, 1208, 1140 and 995 em-I. It was reported that the characteristic absorption bands of thiouram disulphide are 1464, 1424, 1350,1270,1200 and 1140 cm- I (Pouchert, 1970).

1600

1400

1200

1000

800

Wave numbers (crrr-')

Figure 4.45 FTIR spectra ofDDTC and FeD3

FTIR reflection spectra of diethyl dithiocarbamate adsorption on pyrrhotite at pulp pH= 7.0 are demonstrated in Fig. 4.46. From Fig. 4.46, it can be seen that the characteristic absorption bands of dimmer of diethyl dithiocarbamate at 1468, 1425,1350,1269,1201,1139,1064,1005 and 968 cm- 1 appear on the surface of pyrrhotite, indicating the dominating hydrophobic species on pyrrhotite surface to be disulphide of dithiocarbamate. Further, the effect of pulp potential on the adsorption of dithiocarbamate on pyrrhotite was examined and the results are presented in Fig. 4.47. It follows that at pH = 8.8 the dithiocarbamate adsorption on pyrrhotite is mainly of disulphide independent of potential in the range of 297 - 687 mV due to the occurrence of almost the same disulphide characteristic band. However, the intensity of the IR signals ~ changes at various potential values. It demonstrates the intensity of the IR signals of the characteristic peaks of thiouram disulphide and hence its adsorption on pyrrhotite decreases with the increase of the potential from 297 - 687 mV. At pH = 8.8, flotation recovery and 105

Chapter 4

Collector Flotation of Sulphide Mineralse

thiouram disulphide adsorption on pyrrhotite are decreased with the increase of pulp potential from 297 to 687 mV.

1500

1000 Wave numbers (crrr')

Figure 4.46 FTIR reflection spectra of DDTC adsorption on pyrrhotite (pH = 7.0; DDTC: 5 x 10- 3 mollL; Eh = 377 mY)

The effectof pH on the FTIR signal intensity of dithiocarbamate adsorption on pyrrhotite and pyrrhotite flotation recovery is plotted in Fig. 4.48. The intensity of the IR signal is based on peak 1270 cm- I . It can be seen from Fig. 4.47 that the IR signal intensity corresponds wellto flotation recovery. The optimal flotation pH range is corresponding to the optimal adsorption of thiouram disulphide on pyrrhotite. 5. Adsorption of Diethyl Dithiocarbamate on Jamesonite

The FTIRreflection spectra of lead diethyl dithiocarbamate was reported to show the characteristic absorption bands 900,981,1088,1138,1200,1248,1320,1351, 1398 and 1460 cm- I (Pouchert, 1970). The FTIR reflection spectra of sodium diethyl dithiocarbamate adsorption onjamesonite at pH= 7.0 are shown in Fig. 4.49. It canbe seen thatthecharacteristic absorption surface is leaddiethyl dithiocarbamate. Further, the effect of pulp potential on the adsorption of diethyl dithiocarbamate on jamesonitewas examined and the results are shown in Fig. 4.50. At pH = 4.7 the diethyl dithiocarbamate adsorption on jamesonite is mainly of lead diethyl dithiocarbamate independent of potential in the range of 485- 680 mV due to the occurrence of almost the same lead diethyl dithiocarbamate characteristic band. However, the intensity of the IR signals changes at various potential values. It demonstrates the intensity of the IR signals of the characteristic peak lead diethyl 106

4.3

Q)

o

5o

Surface Analysis

390mV 0 0

lr)

r-... r-...

Q)

c;:: Q)

~

0

~

.-

.-

447mV

00

r---:

\0 0

\0

lr)

0 0

-r>

0

r---:

0\

0'\

00

0

M

\0 N

lr)

r-...

r-

0

\0

r-

0 r-

687mV

0

N

~

1500

1000 Wave numbers (crrr l)

Figure4.47 FTIR reflection spectra ofDDTC adsorption on pyrrhotite at different pulp potential (pH = 8.8; DDTC: 5 x 10- 3 mol/L) 100 80

,0

'00 ~

~

---c

I:

E

60

.s ~

Q)

> 0 u

Q)

40

I:

01)

'C;;

Recovery -+- FTIR signal intensity -0-

~

20

0

2

4

6

pH

8

~

E= ~ 10

12

14

Figure 4.48 The effect of pH on the FTIR signal intensity of DDTC adsorption on pyrrhotite and pyrrhotite flotation recovery

107

Chapter 4

Collector Flotation of Sulphide Mineralse

96, - - - - - - - - - - - - - - - - - - - - - - - - , 95 94

~ 93

'8 92

ya 91

~ 90 cG

89 88

87 86

~

L---

1600

1400

__L._

_.L.._

1200 1000 Wave numbers(crrr ')

__L.

800

__J

600

Figure 4.49 FTIR reflection spectra ofjamesonite adsorption DDTC (DDTC: 5 x 10- 3 mol/L; pH = 7.0; E h = 385 mY)

485mV

N

~

v) ~

u

~o

575mV~

~

~

~

r-

\0

v)

00

620mV:!

1600

N

~

1400

1200 1000 Wave numbers(crrr ')

800

600

Figure 4.50 FTIR reflection spectra of DDTC adsorption on jamesonite at different pulp potential (pH = 4.7; DDTC: 5 x 10- 3 mol/L)

dithiocarbamate and hencelead diethyl dithiocarbamate adsorption on jamesonite decrease with the decrease of the pulp potential from 485- 680 mV. These results confirm the flotation and voltammograms results that the primary hydrophobic entityon jamesonite is collector salts. 108

4.3 Surface Analysis

4.3.3 XPS Analysis of Collector Adsorption on Sulphide Minerals The X-ray photoelectron spectroscopy (XPS) identification of the products of xanthate sorption at the surface of galena by Laajalehto et al. (1993) showes that lead xanthate in the molecular form is the adsorbed entity. The distribution of xanthate at the surface is irregular and even at low surface coverage the formation of three dimensional aggregates of lead xanthate occurs. Cases et al. (1990, 1991) studied the influence of the grinding media on the adsorption of xanthate on galena and pyrite using XPS and FTIR. They found that only lead xanthate, either monocoordinated or in the three dimensional form was formed on galena surface after dry grinding and that however, wet grinding led to heterogeneous adsorbed layer formed with monocoordinated lead xanthate and dixanthogen at low concentration; stoichiometric or non-stoichiometric lead xanthate, dixanthogen, and the dimmer of the monothiocarbonate for higher concentration. They also find that only dixanthogen is observed on pyrite surface after using stainless steel rods. Whereas, ferric iron xanthate and physically absorbed xanthate ions are formed when iron rods are utilized. The adsorption of xanthate on jamesonite is further determined by using XPS. Figs. 4.51 and 4.52 are respectively the XPS of Pb(4f5) on jamesonite in the presence and absence of ethyl xanthate at pH= 8.8. It has been found that the electron binding energy ofPb(4f5) onjamesonite surface is 138.07 eY, which shifted 5 to Pb(4f ) 137.47 eV with the difference M pb(4f5) . = 138.07 -137.47 = 0.6 eV in the presence of ethyl xanthate. The chemical shifting of electron binding energy 5 of Pb(4f ) suggested the adsorption of ethyl xanthate on jamesonite and the formation of lead xanthate, which further confirmed the results revealed by the UV and FTIR analyses. 40

Pb(4f5)

142.02(138.07) eV

35 30 ,-... ~

0

:=,

25

00-

fr 20 15 10 5

152

150

148

146

144

142

140

138

Bindingenergy(eV)

Figure 4.51 The XPS ofPb(4f5) ofjamesonite at pH = 8.8

109

Chapter 4

Collector Flotation of Sulphide Mineralse

--------, 45. . - - - - - - - - - - - - , : - 5 Pb(4f

40

)

142.37(137.47)eV

35 M' 30 o ....-I

fr

'-'"

25 20 15"'-'--~

lamesonite+KEX

10'-----'--_-'--_-'--_---'-_---'--_-"'--_-"'--_ __'__--' 152 150 148 146 144 142 140 138 Bindingenergy(eV)

Figure 4.52 The XPS of xanthate at pH = 8.8

Pb(4f5)

on jamesonite surface in the presence of ethyl

Figures 4.53 and 4.54 are respectively the XPS of Sb(3d5) on jamesonite in the presence and absence of ethyl xanthate at pH= 8.8. It is obvious that the electron binding energy of Sb(3d5) on jamesonite surface is 529.83 eY, which shifted to Sb(3d5) 529.15 eV with the difference M Sb(3d5) = 529.83-529.15= 0.68 eV in the presence of ethyl xanthate. The chemical shifting of electron binding energy of Pb(3d5) suggested the adsorption of ethyl xanthate on jamesonite and the formation of antimony xanthate. It has been reported that the electron binding energy of Sb(3d5) are, respectively, 529.50 eV in Sb2S3 and 529, 20 eV in Sb2S5 (Wang Jianqi, 1992). And thus, it may be derived that the antimony xanthate salt with high valence antimony is formed on jamesonite. 75. - - - - - - - - - - - - - - - - - - - - - - - - , Sb (3d5 ) 533.78(529.83)eV 70 65 60 55

.-.... o 50

rrl

fr

'-'"

45 40 35 30 25 20

L..-..-_ _-...L-

545

-...L...

540

-...L...

535

----L-_---'

530

Bindingenergy(eV)

Figure 4.53

110

The XPS of Sb(3d 5) on jamesonite surface at pH = 8.8

4.3

Surface Analysis . Sb(3d 5 ) 534.05 (529.15)eV

80 75 70 65 ~

r')

0

.......

60

'-' rJ:l

Q..

Co)

55 50 45 40 35 30

Jamesonite+KEX 545

540 535 Bindingenergy(eV)

530

Figure 4.54 The XPS of Pb(3f 5) on jamesonite surface in the presence of ethyl xanthate at pH = 8.8

Because of the strong hydrolysis property of antimony salt, it is very difficult to prepare antimony xanthate salt to obtain its IR spectrums. Therefore, the formation of antimony xanthate is difficult to be identified by the UV and FTIR analysis, which has been determined by using XPS. Finally, it can be concluded that the interaction mechanism between ethyl xanthate and jamesonite are attributed to the formation of lead and antimony xanthate on the surface in the light of flotation results, voltammogram measurement, UV and FTIR as well as XPS analyses.

111

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

Abstract In this chapter, the depression mechanism of five kinds of depressants is introduced respectively. The principle of depression by hydroxyl ion and hydrosulphide is explained which regulates the pH to make the given mineral float or not. And so the critical pH for certain minerals is determined. Thereafter, the depression by cyanide and hydrogen peroxide is narrated respectively which are that for cyanide the formation of metal cyanide complex results in depression of minerals while for hydrogen peroxide the decomposition of xanthate salts gives rise to the inhibitation of flotation. Lastly, the depression by the thio-organic such as polyhydroxyl and poly carboxylic xanthate is accounted for in detail including the flotation behavior, effect of pulp potential, adsorption mechanism and structure-property relation. Keywords depressant; hydroxyl ion; hydrosulphide ion; cyanide; hydrogen peroxide; thio-organic Modifiers in the flotation of sulphide minerals mainly include depressants and activators. A depressant is defined as a reagent which inhibits the adsorption of a collector on a given mineral or adsorbed on the mineral to make the surface hydrophilic, and includes inorganic depressants such as lime, sodium cyanide, sulphur dioxide, zinc sulphate, sodium sulphide etc., and organic depressants such as sulfhydryl acetic acid, polyacrylamide polymers containing various functional groups etc. In this chapter, roles of depressants in the flotation sulphide minerals will be discussed and some new organic depressants will be introduced.

5.1 Electrochemical Depression by Hydroxyl Ion From the point of view of electrochemistry of flotation, a depressant is, however, defined as a reagent by the addition of which the oxidation of the mineral surface occurs at lower potential than collector oxidation or formation of metal collector salt which may be also decomposed under the conditions given in the discussions which follow. Under these conditions, the mixed potential model becomes one of mineral oxidation and oxygen reduction, the oxidation of the thio collector or the formation of the metal collector is suppressed, and the mineral will remain

5.1 Electrochemical Depression by Hydroxyl Ion

hydrophilic. Thus, hydroxide, sulphide, cyanide and other redox agents are usually used as depressants of sulphide minerals.

5.1.1 Depression of Galena and Pyrite The underlying principle of depression by hydroxyl ion is that for each concentration of collector there is a pH below which any given mineral will float and above which it will not float. This is the so-called critical pH which can be determined according to the relative extent of oxidation of the mineral surface and collector or the formation of the metal collector salt. For the case that the hydrophobic entity is disulphide, the mineral will be depressed when the reaction of the type (2-3) or (2-4) occurs before the reaction (1-3). Thus for the pyrite /diethyl dithiophosphate (DTP) system, pyrite will be depressed if the oxidation reaction (5-1a)

EO= 0.402(V) { E = 0.402 - 0.07473pH + 7.87 x 10-3Ig[SO~-] h1

(5-1b)

takes place prior to the oxidation reaction (5-2a)

Eh 2 = 0.252 - 0.059Ig[DTP-]

(5-2b)

The critical condition is (5-3a) Assuming that the sulphur could be oxidized to sulphate at overpotential 0.5 V and the concentration of sulphate is 10- 6 mol/L, then Eqs. (5-1b), (5-2b) and (5-3a) become 0.8548 - 0.07473pH ~ 0.252 - 0.059Ig[DTP-]

(5-3b)

Thus, the critical pH above which pyrite will not float is H d ~ 0.6028 + 0.0591g[DTP-] P Py ~ 0.07473 .

(5-4)

In the case of the galena/diethyl/dithiophosphate system, if the reaction Eq. (2-22a) occurs before the following reaction 2PbS+4DTP-+3H 20=2Pb(DTP)2+ s2oi-+ 6H++8e-

(5-5a) 113

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

E O= 0.245(V) {

(5-5b) E; = 0.245 - 0.04425pH - 0.0295lg[DTP-] + 0.007375lg[S20~-]

where EO is calculated in the light of the standard free energy change of reaction (5-5a) and the solubility product of Pb(DTP)2 is 7.5 X10- 12 from reference (Karkovsky, 1957). The critical pH condition determined by the reaction Eqs. (2-22) and (5-5) is -] H d ~ 0.366 + 0.02951g[DTP-] = 12.43 + 1 [ P Ga 0.02945 g DTP

(5-6)

If lead dithiophosphate is decomposed by the following reaction Pb(DTP)2 + 20H- = K

HPbO; + 2DTP- + H+

(5-7)

= 7.73 X 10-12

Then, galena will be depressed. The critical pH determined by decomposition reaction is defined by (5-8) Similar results are obtained for pyrite/ethyl xanthate from the Eqs. (5-1) and (1-3). H d ~ 0.9418+0.059lg[EX-] =12.24+0.791 [ -] P Py 0.07473 g EX

(5-9)

For the galena/ethyl xanthate system critical pH determined by Eqs. (2-22) and (4-12b) is pH~a ~ 15.2 + 19[EX-]

(5-10)

The critical pH determined by the decomposition of lead xanthate is (5-1la) K = 2.1X10- 17

2 _ { pH = 12.89 +-lg[EX ] 3

(5-l1b)

Therefore, the critical pH of hydroxyl depression of pyrite and galena could be calculated using Eqs. (5-4) to (5-8) or Eqs. (5-9) to (5-11). The results are given in Fig. 5.1 for the diethyl dithiophosphate system and in Fig. 5.2 for the ethyl xanthate system. Also shown in the same figures are the so-called contact curves (solid lines) from Sutherland and Wark (1955). The dashed lines are the calculated 114

5.1 Electrochemical Depression by HydroxylIon

theoretical contact curves. It can be seen from Fig. 5.1 and Fig. 5.2 that the calculated collector concentration critical pH curves are in reasonable agreement with those results obtained form contact curves or flotation data. It shows that the hydroxyl depression of pyrite is attributed to its greater ease of oxidation to iron hydroxide and oxy-sulphur species which renders pyrite surface hydrophilic than dithiophosphate oxidation to dithiophosphatogen. 4

--Pyrite - - - - Eq.(5-4)

~

c

I

.......... Galena

3

--Eq.(5-7) - - - Eq.(5-8)

15~

I

c-

I

~7 co

I

.9 ~ 2 0 ......

(,)

C

'-"

I

o

i

(,)

..

o

2

4

6

'

8

pH

10

12

Figure 5.1 Relationship between concentration of sodium diethyl dithiophosphate and critical pH; dashed line is the resultcalculated basedon Eqs. (5-4),(5-7)or (5-8); solid line is the data from Sutherland and Wark (1955) -3 r - - - - - - - - - - - - - - - - - ,

/0

/°

Pyrite

-4

/0

fi

/0

i

-5

/'0

:.............:/ Galena

0

-6 '-------L_--L_----L-_--L-_--I.-_-'--~ 6

7

8

9

pH

10

11

12

13,

Figure 5.2 Relationship between concentration (mol/L) of potassium ethylxanthate and critical pH; dashed line is the calculated results and symbols are the results from Fuerstenauet al. (1968)

Janetski et al. (1977) used ···voltammetry to study the oxidation of pyrite electrode in solution at different pH in the absence and presence of ethyl xanthate to demonstrate that the oxidation of pyrite itself increases as the pH is increased. At high pH condition, the oxidation of pyrite occurs at a potential cathodic to that for xanthate oxidation and hence, only the mineral will be oxidized at the mixed potential and flotation will be depressed. 115

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

Lime is a special kind of hydroxyl depressant and usually used for depressing pyrite in plant, which has stronger depression on pyrite than other alkaline depressants at the same pH condition (Fuerstenau, et aI., 1968; Hu et aI., 1995). Figure 5.3 shows that at pH = 12 modified by CaO and NaOH, galena is fairly floatable and pyrite is depressed. CaO is a strongerdepressant than NaOH. 100.----------------------, 90 PbS+NaOH

80 -. 70

?ft.

"6 Q)

PbS+CaO

60

:

>

8 50

~

40 30

zn~

20

IO~~ e:= FeS2+CaO

01.-----1.....-----1.....-----""'-------' 0.0004 0.0008 0.0012 Collectorconcentration(mol/L)

Figure 5.3 Flotation recovery of galena and pyrite as a function of butyl xanthate concentration by using lime and sodium hydroxide at pH = 12

Figure 5.4 shows the anodic oxidation of pyrite at different pH modified by CaO and NaOH. Obviously, the oxidation of pyrite is more rapid in CaO solution than in NaOHsolution at the same pH. The calcium species in oxidized pyrite surface in the presence ofCaO has beenidentified using XPSas shown in Fig. 5.5. Thepyrite surface is oxidized to form Ca(OH)2, CaS04 andFe(OH)3 in thepresence ofCa lime. 300.-------------------, CaO(mg/L) pH

200

e ~~ 100 o

1:320 12.2 2: 150 10.8 3: 50 9.6 4:0 6.8 5:NaOH,12

0.2 0.4 Potential(V, SHE)

3

0.6

Figure 5.4 Voltammograms for a pyrite electrode in solutions at different pH conditions modified by CaO and NaOH (Linear potential sweeps at 20 mV/s)

116

5.1 Electrochemical Depression by Hydroxyl Ion

CaO 320 mg/L Pyrite

344

346

348

350

352

354

Binding energy(eV)

Figure 5.5 The extended XPS spectrum of Ca of pyrite surface treated in CaO solution

Voltammograms for a galena electrode in strong alkaline solution are presented in Fig. 5.6. It demonstrates that only the anodic oxidation of galena surface occurs in strong alkaline solution due to the reaction ofEq. (2-22) or (2-23) at the lower concentration of butyl xanthate or even high concentration of ethyl xanthate and thereby galena will be depressed. If the concentration of butyl xanthate is high enough to 2 x 10-3 mol/L, an evident anodic current which may be attributed to the formation of lead butyl xanthate occurs by the reaction of the type (4-12b).

BX: 10-4 mOI_/L

~~

BX: 2x 10- 3 mol/L EX: zx 10- 3 mol/L ~----

No collector

~:.-;.;.;..~==;;;;;;;...--

-0.8 -0.6 -0.4 -0.2

0 0.2 0.4 0.6 Potential (V)

0.8

1.0 1.2

Figure 5.6 Voltammograms for a galena electrode in strong alkaline solution (CaO: 320mg!L, pH = 12.2)at different concentration of xanthate (scanrate. 20 mV/s)

5.1.2 Depression of Jamesonite and Pyrrhotite With ethyl xanthate (IO-4 moVL) as a collector, the flotation recovery ofjamesonite and pyrrhotite as a function of pH is presented in Fig. 5.7 with NaOH as pH modifier and in Fig. 5.8 with CaO as pH modifier. It can be seen from these two figures that both jamesonite and pyrrhotite exhibit good flotation response at t:

117

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

pH < 12 in case of NaOH modifying and at pH < 11 in case of CaO modifying pH. Jamesonite with recovery above 90% has better floatability than pyrrhotite with recovery around 80%. CaO appears as a stronger depressant than NaOH on jamesonite and pyrrhotite. 100 80 ..-.... ?J(

'6

60

o (I)

40

(I)

;> 0

~

--+- Jamesonite -

20

o

2

Pyrrhotite

4

6

pH

8 10 12 14

Figure 5.7 Flotation recovery of jamesonite and pyrrhotite as a function of pH modified with NaDH 100 80 ..-.... ?J(

'6

60

u (I)

40

(I)

;> 0

~

--+- Jamesonite -

20

Pyrrhotite

O'-----'--_""""--~_""""--___'_

7

8

9

10 11 pH

12

___J

13

14

Figure 5.8 Flotation recovery of jamesonite and pyrrhotite as a function of pH modified with CaD

5.1.3 InterfacialStructure of Mineral/Solution in Different pH Modifier Solution 1. Interfacial Structure of Marmatite/Solution In 0.1 mol/L KN0 3 solution, pH is adjusted to 10.0 by NaOH, Ca(OH)2 and Na2C03. The Tafel curves of marmatite electrode in the above solutions are determined as shown in Fig. 5.9. It follows from Fig. 5.9 that the electrochemical parameters of corrosive potential and current are almost not affected by the pH 118

5.1

Electrochemical Depression by Hydroxyl Ion

modifiers. The corrosive potential in Na2C03 and Ca(OH)2 solutions only change 3 mV and 13 mV negatively compared to that in NaOH solution, "v, effect" of Stem double electric charge layer occurs. So the pH modifier affects the structure of the double electric charge layer. 600...--------------------, pH=10.0 500 400

1: NaOH

2: Na2C03 3: Ca(OHh

>' 300 E

~

200 100

o -9.5 -9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5

IgIa

Figure 5.9 Polarization curves of mannatite electrode in 0.1 mol/L KN0 3 solution with different pH adjusting media (unit of fa: A/cm 2)

If the effect of potential lfIl in diffusion layer is considered, then the electrochemical polarization equation is: 11

'fa

RT RT z-nfi = en ---lnnFKc +--lnf + - - l f I v: nfiF R nfiF a nfi 1 z-nfi RT =constant+--lfIl +--lnfa nfi nf3F

(5-12)

The relationship between the over-potential and 19 fa will deviate from the Tafel linear area due to the medium affecting the diffusion layer. The lfIl effect will gradually disappear and the polarization curves separate each other obviously when the potential is far from zero electric charge potentia1. This is the reason that CO~- and Ca(OH)+ ions have some surfactant action compared with OHion to form characteristic adsorption more easily and to bring about the change of the capacitance of the double electric charge layer. Figure 5.10 is EIS of marmatite electrode in O.lmol/L KN03 solution with different pH modifiers at open circuit potential. This EIS is very complicated. Simple equivalent circuit can be treated as the series of electrochemical reaction resistance R, with the capacitance impedance C, = (nF rO)/(K~) resulting from adsorbing action, and then parallel with the capacitance Cd of double electric

119

Chapter 5

Roles of Depressants in Flotation of Sulphide Minerals

11

pH=lO.l

10

~NaOH - 0 - Na2C03 ....... Ca(OH)2

9

8

a

7

~

E 6 N"" 5 4 3 2 1

o

10

20

30

40

50

60

70

80

Zre(kQ)

Figure 5.10 EIS of marmatite electrode in 0.1 mol/L KN03 solution with different pH adjusting media

charge layer. Obviously, Cd(Ca(OH)2) > Cd(Na2C03) > Cd(NaOH). It reveals the effect of the pH modifiers on the interfacial structure of marmatite/solution: Ca(OH)2> Na2C03 > NaOH. Usually, a cation is easy to hydrolyze and an anion is not so easy to hydrolyze that its characteristic adsorption forms. Adsorbed anion will enter inside H20 polar layer and push aside H20 to form tight structure on the mineral. The effective thickness of the tight layer will decrease and its capacitance will increase. Ca(OH)2 in solution is very easy to ionize into Ca(OH)+, which is not easy to hydrolyze and is able to form strong characteristic adsorption on the surfaceof the mineral. 2. Interfacial Structure of Jamesonite/Solution

In 0.1 mol/L KN0 3 solution, pH is adjusted to 9.98 by NaOH, Ca(OH)2 and Na2C03 respectively. Electrochemical impedance spectroscopy (EIS)of jamesonite electrode in 0.1 mol/L KN0 3 solution with different pH adjusting was measured and the results are given in Fig. 5.11. Because RNaOH > RCa(OHh' Cd = 11(mR), then Cd(Ca(OH)2)< Cd(NaOH). It suggests that Ca(OH)+ has stronger tendency to adsorb ontojamesonite surfacethan OH-. Ca(OH)2 is a more efficientdepressant than NaOH in the flotation of sulphide minerals. The Tafelcurves of jamesonite electrode in the above solutions are determined and shown in Fig. 5.12. It may be seen from Fig. 5.12 that: (1) The corrosive potential of jamesonite in NaOH, Ca(OH)2 and Na2C03 solution are 45 mY, 57 mV and 63 mV respectively. Three kinds of pH adjustors do not influence much on their corrosive potential, only a little positive moving occurs. It indicates that these adjustors have a similar effect on jamesonite electrode process. The promoting actions of these adjustors for the cathodic processbecome slightly strong in the order of NaOH, Ca(OH)2 and Na2C03. 120

5.1 Electrochemical Depressionby Hydroxyl Ion

6000

•

5000

•

4000

a eN- 3000 2000 1000 0

•

•. .II-.o

-

-

-

1000 2000 3000 4000 5000 6000 7000 Zre(n)

Figure 5.11 EIS of jamesonite electrode in 0.1 mol/L KN0 3 solutionwith different pH adjustingmedia (pH = 9.98; square dot: NaOH; ellipse dot: Ca(OH)2) 400 Solid line: NaOH Dash line: Na2C03 Dot line: Ca(OH)2

300

200

>E 100

- - - - - ----r~m-;..:::.;::.::. - -.::.:':

~

.

~"""

0

.......

-100 -200

-11

-10

-9

-8 -7 IgIa

-6

-5

-4

Figure 5.12 Tafel curvesof jamesonite electrode in 0.1 mol/L KN0 3 solution with differentpH adjustingmedia (pH = 9.98, unit of fa: A/cm2)

(2) The density of the corrosive current of jamesonite in NaOH solution is basically the same as that in Ca(OH)2 solution, but it is minimal in Na2C03 solution, about a fraction of the fourth of the former. There are obvious appearances of passivation and its breaking-down in strong polarization area in NaC03 solution' Because CO~- ion is easier to form insoluble alkaline carbonate than OH- ion, the carbonate salts are passive on the mineral surface to inhibit oxidation reaction. (3) In the Tafel linear area, the anodic slope in the Ca(OH)2 solution is slightly bigger than that in NaOH solution. It results from characteristic adsorption as the surface activity of Ca(OH) + ion is bigger than that of OH- ion. 121

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

Therefore, the reaction rate of corrosive oxidation of jamesonite and its adsorption characteristics are different in different media of adjusting pH at the same pH condition. The oxidation rate of jamesonite is minimal in Na2C03 solution. It is basically the same in Ca(OH)2 solution as in NaOH solution, but Ca(OH)+ is more easy to adsorb onjamesonite surface.

5.2 Depression by Hydrosulphide Ion The depression by hydrosulphide ion is in a similar manner as hydroxyl depression, i.e. there is a critical pH for each HS- ion concentration at a constant xanthate concentration above which no flotation is possible. In the case that the hydrophobic entity is disulphide, the mineral will be depressed when the reaction (3-5a) or (3-6a) occurs before the reaction (1-3). Thus for the pyrite /ethyl xanthate system, pyrite will be depressed if the oxidation reaction (3-5a) takes place prior to the oxidation reaction (4-35). The critical condition is

s, [Eq. (3-5b)] < e, [Eq. (4-35b)]

(5-13)

Assuming that sulphur could be oxidized to sulphate at overpotential 0.5 V and the concentration of sulphate is 10- 6 mollL, then Eq. (5-13) become 0.5953 - 0.066pH - 0.007375 Ig[HS-] < - 0.06 - 0.059Ig[Er]

(5-14)

Thus, the critical pH above which pyrite will not float is Hd P Py

~ /"

0.6553 + 0.059Ig[EX-] - 0.007375Ig[HS-] 0.066

(5-15)

For the galena/ethyl xanthate system, if the reaction Eq. (3-5a) occurs before the reaction (4-12a), the critical pH condition determined by the reaction Eqs. (3-5b) and (4-12a) is p

Hd

~

Ga /"

0.071 + 0.01475Ig[EX-] - 0.007375Ig[HS-] 0.015

(5-16)

Therefore, the critical pH of hydrosulphide ion depression of pyrite and galena could be calculated using Eqs. (5-15) and (5-16). The results are plotted in Fig. 5.13. The same figure is the results of contact curves reported by Wark and Cox (1933). It can' be seen from Fig. 5.13 that the critical pH condition defined by electrochemical equilibriums of Eqs. (5-15) and (5-16) is correlated reasonably well with the contact curves. The calculated concentration of sodium sulphide required for preventing the interaction between xanthate and galena or pyrite at 122

5.3 Electrochemical Depression by Cyanide

various pH is quite close to the results from the contact curves reported by Wark and Cox (1933).

:3'

100,.-----------------, Contact possible 90 below curves KEX:1.56xI0-4 mol/L 80

bb 70

8

§

60

~

50 40

~

30

.~

go

\

\

\

~

\

;z; 20

10

o

Pyrite - - - Galena --- Eq.(5-14) ........ Eq.(5-15) \

\

\

2

4

\

6

8

10

12

14

pH

Figure 5.13 Relationship between pH and concentration of sodium sulphide necessary to prevent contact at surfaces of galena and pyrite

Janetski et aL (1977) used voltammetric method to study the electrochemical behavior of a pyrite electrode in ethyl xanthate solution containing various concentration of sodium sulphide. They observed an additional anodic wave due to the oxidation of the dissolved sulphide species present and that the wave appeared at potential cathodic to xanthate oxidation. Therefore, they concluded that the presence of sulphide in solution introduced an anodic process which occurred in preference to xanthate oxidation and hence dixanthogen would not be formed and the pyrite would not be rendered floatable.

5.3 Electrochemical Depression by Cyanide The depression of pyrite by cyanide has been considered from the electrochemical viewpoint, the formation of ferric ferrocyanide in the surface of pyrite which was perhaps responsible for cyanide depression was proposed by Ball and Richard (1976) to occur by the following electrochemical reaction. (5-17) This equation represents the overall reaction which must occur in several steps. The mineral must produce some ferrous ion in solution which reacts with C~ to form Fe(CN):-. Using measured oxidation potential and pyrite solubility values, Eligillani and Fuerstenau (1968)delineated the stability domains of the compound 3 2 Fe4[Fe(CN)6]3, Fe(OH)3, Fe +, Fe +, HCN, for various concentration of KCN in 123

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

the Eh-pH diagram that the regions of no flotation are predicted corresponding to the stabilityfields ofFe4[Fe(CN)6]3 and Fe(OH)3. Janetski et aI. (1977) also studied the behavior of a pyrite electrode in a solution of cyanide concentration in the absence and presence of xanthate using voltammetric technique. They reported that on increasing the concentration of cyanide at constant pH and xanthate concentration, the oxidation wave of xanthate is shiftedto more anodicpotential; indicating that the presenceof cyanide, which may react with the mineral surface to form an insoluble iron cyanide complex will result in the inhibition of the electrochemical oxidation of xanthate and the depression of pyrite.

5.4 Depression by Hydrogen Peroxide Many oxidizing agents such as hydrogen peroxide (H202) , sodium hypochlorite (NaCIO), potassium permanganate, potassium chromate have been used as depressants for pyrite, arsenopyrite and galena (Beattie and Poling, 1988; Shimoiizaka et aI., 1976; Hoyack and Raghavan, 1987; Nagaraj et aI., 1986; Wang, 1992). Figures 5.14 and 5.15 demonstrate the influence of pulp potential and concentration of hydrogen peroxideon the flotation of galena and chalcopyrite. It can be seen from Fig. 5.14 that chalcopyrite has high flotation recovery in the potential range of 0.4- 0.7 V modifiedby hydrogen peroxide and the floatability of galena sharply decreases with the potential enhancing. Figure 5.15 shows that galena is completely depressed and chalcopyrite remains floatable if the concentration of H202 is greater than 10-3 mollL, suggesting that the conditions of flotation separation of chalcopyrite from galena in pulp potential is greater than 0.,65 V at pH> 7.3 with 10-3 mol/LH202• 100.------------------. 90 80 ~

~

'6 ~

70 60 50

8 40 Q)

~

---- Chalcopyrite Galena

30

20

-+-

10

o

'-------...------'--A-----'-------'-------'

0.45

0.55

0.65 0.75 Potential(V, SHE)

0.85

0.95

Figure 5.14 Flotation recovery of chalcopyrite and galena as a function of pulp potential modified by hydrogen peroxide (pH = 9.5; Wang, 1992) 124

5.5 Depression of Marmatite and Pyrrhotite by Thio-Organic Depressants 100 80

- 0 - Galena --- Chalcopyrite

,,-..,

~

'6

60

(])

>

0

o (])

cG

40 20

o

10

20 30 40 Cone. of H202 (10-4 mol/L)

50

60

Figure 5.15 Flotation recovery of chalcopyrite and galena as a function of concentration of hydrogen peroxide (Wang, 1992)

The strong depressing action of H202 on galena may be attributed to its strong oxidizing action on lead xanthate in galena surface giving rise to the oxidation and decomposition of lead xanthate by the reaction of (5-18):

[Pb(EX)2]ads + H202~Pb(OH)2 + (EX)2+ Ze

(5-18)

EO = -0.94(V) Therefore, at certain pH and H202 concentration, the complete decomposition of lead xanthate preadsorbed on galena renders galena surface hydrophilic and depression of galena, whereas the dixanthogen prefixed on chalcopyrite remains stable which confers chalcopyrite surface hydrophobic and floatable, which was proved by voltammogram method (Wang, 1992).

5.5

Depression of Marmatite and Pyrrhotite by ThioOrganic Depressants

Organic depressants were paid attention to by many scholars. The depressions of small molecular organic depressants in sulphide ore flotation were reported by some authors (Zhao et aI., 1988; Lin et aI., 1991; Cheng and Liu, 1994; Xu and Huang, 1995). The results show that, in sulfhydryl acetic acid, the sulfhydryl group acts as its mineral-philic group, while carboxyl group as its hydrophilic radical. Because of the relatively strong reducibility of sulfhydryl acetic acid, its inhibiting behavior fall into two parts. It can produce a relatively negative pulp potential to remove the xanthate adsorbed on the surface of the sulphide minerals or it is fixed on the surface of the solid by means of chemical adsorption. The stronger the reducibility, the better the inhibiting ability. Polymers are well-known depressants of both sulphide and non-sulphide minerals (Li et aI., 2001; Xiong et aI., 2004). Sun Wei et aI. found that the new organic polymer 125

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

depressant RC has a strong depression effect on pyrite and pyrrhotite. Nagaraj (1982) has indicated that polyacrylamide polymers containing various functional groups can depress iron sulphide minerals. In particular, a recent review by A. Boulton et al[ll] showed that low molecular weight PAM polymers could be used to separate efficiently copper-activated sphalerite from pyrite by flotation in the presence of isobutyl xanthate. Xuan Daozhong used combined depressant calcium chloride with sodium humate to successfully separate the mixture of marmatite and pyrrhotite which was activated by cupric ions with potassium butyl-xanthate as a collector. Structure and Reactivity of Organic Depressant on Flotation have also been studied by many authors (Baldauf and Schubert, 1980; Wang, 1983; Cheng et al., 1998). The basic requirements of organic depressant is that there must be several (at least two) polar radical in molecular structure, which has a polar radicals to interact selectively with mineral surface, which is definitely stronger than the collector. It must have other polar radicals that render mineral surface hydrophilic. The effect of DMPS on the flotation recovery of pyrrhotite and marmatite in the presence and absence of CUS04 with butyl xanthate is shown in Fig. 5.16. It follows that the flotation of pyrrhotite and marmatite is greatly affected by DMPS addition. In the absence of cupric ion the recovery of pyrrhotite hardly exceeded 40%. At pH = 2, the recovery of marmatite is more than 90%, but the recovery sharply decreases to below 20% with pH increasing. These results show that pyrrhotite and marmatite can not be separated in the absence of cupric ion with DMPS as depressant and xanthate as a collector. In the presence of cupric ions, marmatite flotation improves under wide pH condition. The flotation of pyrrhotite is activated only around pH = 2. The results demonstrate that flotation separation of copper-activated marmatite from pyrrhotite is possible in the presence of butyl xanthate and DMPS. 100..----------------,

80

~ 60

~;> o

as 40

~

20

2

4

8

6

10

12

pH

Figure 5.16 Flotation recovery of mineral as a function of pH using [KBX] = 10-4 mol/L as a collector and [DMPS] = 2 x 10-4 mol/L as a depressant (I-pyrrhotite; 2-marmatite; 3-pyrrhotite+ [CUS04, 10-4mol/L]; 4-marrnatite+ [CUS04, 10-4mol/L])

126

5.5 Depressionof Marmatite and Pyrrhotiteby Thio-Organic Depressants

At different pH, the flotation response of marmatite and arsenopyrite is presented in Fig. 5.17 and Fig. 5.18, using 125 mg/L glycerin-xanthate as a depressant and 1 x 10- 4 mol/L butyl xanthate as a collector. At pH < 7, marmatite has a good floatability and glycerin-xanthate exhibits little effect on the flotation of marmatite. However, the flotation recovery of marmatite falls down sharply with pH increasing. At pH > 9, it is completely depressed in the presence and absence of glycerin-xanthate. Arsenopyrite shows a good flotation at pH < 4 in the absence of depressant, glycerin-xanthate has a stronger depression effect on arsenopyrite from pH = 4 - 12, the flotation recovery is below 20%. It indicates the possibility of flotation separation of marmatite from pyrrhotite with butyl xanthate as a collector, cupric ion as an activator and glycerin-xanthate as a depressant.

•

100"'--~-------------'

80

~ C Q)

- - Mannatite _ Mannatite+glycerin -xanthate

60

;;..

o

g 40

~

20

o

2

4

6

pH

8

10

12

14

Figure 5.17 Flotationrecovery of marmatite as a function of pH in the presence and absenceof glycerin-xanthate 100

__ Arsenopyrite _ Arsenopyrite+glycerin -xanthate

80 ..-.. ~

'6

60

Q)

>

0

o Q) ~

40 20

o

2

4

6

pH

8

10

12

14

Figure 5.18 Flotation recovery of arsenopyrite as a function of pH in the presence and absenceof glycerin-xanthate

127

Chapter 5 Roles of Depressants in Flotationof SulphideMinerals

The FTIR reflection spectra of organic depressant glycerin-xanthate (HO)n-R O-CSSNa is given in Fig. 5.19, It shows the stretchingvibrationbands of the c=o at 1414 cm- I and 1041 cm- I , O-H at 3358 cm- 1, the dissymmetry stretching vibration bands of the C-H at 2942 em -1. The stretching vibration bands of the C=S at 1631 cm- I , 1149 cm- I , and 1043 cm- 1 are the most important vibrations of xanthate group.

1:Glycerin 2: Glycerin-xanthate

4000

3500

3000

2500

2000

1500

1000

500

Wave numbers (crrr')

Figure 5.19 Infraredspectrumof organicdepressantglycerin-xanthate

4000 3500 3000 2500 2000 1500 1000

500

Wave numbers (crrr ')

Figure 5.20 FTIRspectra of marmatite in theabsence of reagents and in thepresence of CUS04' butyl-xanthate, glycerin-xanthate (1: marmatite; 2: marmatite+CUS04 + butyl-xanthate; 3: marmatite + glycerin-xanthate + CUS04 + butyl-xanthate)

The FTIR reflection spectra of marmatite in the absence of reagents and in the presence of CUS04, butyl-xanthate, glycerin-xanthate are shown in Fig. 5.20. It 128

5.6 Roleof Polyhydroxyl andPolyCarboxylic Xanthate in the Flotation of Zinc-Iron Sulphide I

follows that the absorption bands 1461, 1196, and 1030 cm- can be observed, after the mannatite was activated by Cu2+ and reacting with butyl-xanthate. It suggests that the metal butyl-xanthate and dixanthogen molecule absorb on the surface of marmatite. In the FTIR spectra of marmatite reacted with glycerin2 xanthate in the presence of Cu + and butyl-xanthate, the characteristic absorption I 1 bands at 1460 cm- ,1191 cm- can be observed and the stretching vibration band of the -OH at 3200 - 3500 em-I does not exist. The results show that glycerinxanthate can not prevent butyl-xanthate absorbing on the surface of marmatite, which remains hydrophobic and with good flotation.

5.6 Role of Polyhydroxyl and Poly Carboxylic Xanthate in the Flotation of Zinc-Iron Sulphide 5.6.1 Flotation Behavior of Zinc-Iron Sulphide with Polyhydroxyl and Polycarboxylic Xanthate as Depressants With butyl xanthate (1.0x 10- 4mo1!L) as a collector and 2,3-dihydroxyl propyl dithiocarbonic sodium (GX2) as a depressant, the flotation recovery ofmarmatite, arsenopyrite and pyrrhotite is given in Fig. 5.21 as a function of pH. 100r--------------., BX: 10-4 mol/L 80

20

o

2

4

6

pH

8

10

12

14

Figure 5.21 Flotation recovery of mannatite, arsenopyrite and pyrrhotite as a function of pH (BX: 1.0 x 10- 4 mollL, GX2: 120 mg/L)

It can be seen from Fig. 5.21 that the flotation recovery of marmatite, arsenopyrite and pyrrhotite decreases with the increase of pH. In the pH range of 4-8, 2,3-dihydroxylpropyl dithiocarbonicsodium (GX2) exhibits stronger depressing action on arsenopyrite and pyrrhotite than on mannatite. The recovery of marmatite is above 70% and the recovery of pyrrhotite and arsenopyrite is below 35%. 129

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

At pH = 6, the flotation recovery of marmatite, arsenopyrite and pyrrhotite as a function of depressant dosage GX2 is given in Fig. 5.22. With the increase of GX2 dosage, the flotation recovery of these three minerals decreases. However, marmatite remains with reasonably high flotation recovery of above 70%, and arsenopyrite and pyrrhotite exhibit poor flotation with recovery of below 35% when the concentration of GX2 is above 120 mg/L. It indicates the possibility for flotation separation of marmatite from arsenopyrite and pyrrhotite by using 2,3-dihydroxyl propyl dithiocarbonic sodium as a depressant and butyl xanthate as a collector. 100r-----------------, pH=6.0 BX:10-4mol/L 80

-Marmatite -+- Arsenopyrite -.- Pyrrhotite

20

o

20

40

60 80 100 120 140 160 180 Cone. of GX2(mg/L)

Figure 5.22 Flotation recovery of marmatite, arsenopyrite and pyrrhotite as a function of depressant dosage 100 pH=6.0 -Marmatite -+- Arsenopyrite -.- Pyrrhotite

80 ,.-...

'$. 60

'6 Q)

:> 0 o Q)

cG

40 20

o

2

4

6

8

10

12

14

pH

Figure 5.23 Flotation recovery of marmatite, arsenopyrite and pyrrhotite as a function of pH (BX: 1.0 x 10- 4 mol/L, TX4: 120 mg/L)

Figure 5.23 and Fig. 5.24 demonstrate the flotation results of marmatite, arsenopyrite and pyrrhotite with butyl xanthate (1.0x 10-4 mollL) as a collector 130

5.6 Roleof Polyhydroxyl andPolyCarboxylic Xanthate in theFlotation of Zinc-Iron Sulphide

and (l-carbonic sodium-2-acetaic sodium) propanic sodium dithio carbonic sodium (TX4) as a depressant. It is obvious that similar to GX2, TX4 has stronger depression on arsenopyrite and pyrrhotite than on marmatite in the pH range of 2-6, at which the flotation recovery of marmatite can be above 70%, and that of arsenopyrite and pyrrhotite is below 30%. At pH = 6, with the increase of TX4 dosage, the flotation recovery of these three minerals decreases. However, marmatite still remains with reasonably high flotation recovery of above 60%, and the flotation of arsenopyrite and pyrrhotite is almost depressed with recovery of below 20% when the concentration ofTX4 is above 120 mg/L. It also indicates that the flotation separation of marmatite from arsenopyrite and pyrrhotite may be possible with TX4 as a depressant and butyl xanthate as a collector. 100..----------------. pH=6.0 BX: 10-4mol/L

80

~ 60

'6

-Marmatite ...... Arsenopyrite

Q)

;>

o

g 40

........ Pyrrhotite

~

20

o

20

40

60

80

100 120 140 160

Cone. of TX4(mg/L)

Figure 5.24 Flotation recovery of marmatite, arsenopyrite and pyrrhotite as a

function of depressant dosage

5.6.2 Effect of Pulp Potential on the Flotation of Zinc-Iron Sulphide in the Presence of the Depressant The influence of pulp potential on the flotation of marmatite, arsenopyrite and pyrrhotite with 10-4 mol/L butyl xanthate as a collector in the presence of 150 mg/L 2,3-dihydroxyl propyl dithiocarbonic sodium (GX2) has been tested. Taking the flotation recovery to be 50% as a criterion, above which the mineral is considered to be floatable and otherwise not floatable, the upper and lower potential limits of the flotation of marmatite, arsenopyrite and pyrrhotite at different pH are presented in Fig. 5.25 and Table 5.1. It is evident that marmatite is floatable in some range of potential at various pH, whereas arsenopyrite and pyrrhotite are not floatable in the corresponding conditions. It suggests that the flotation separation of marmatite from arsenopyrite and pyrrhotite may be 131

Chapter 5

Roles of Depressants in Flotation of Sulphide Minerals

accomplished with butyl xanthate as a collector and GX2 as a depressant by modifying pH and pulp potential. At acidic pH region, the potential range of marmatite flotation is wider, which is suitable for the flotation separation of marmatite from the other two minerals. 900.-------------------. 800 700

W 600 :r: ir:

? 500 E

~ 400

300 200

100

L...--L.---'----L..---L..-.I.-....I..--'-----L...-......I..-...r...........--I-----L...---L.-....L....-...J"---J----J

3

4

5

6

7

8

9

10

11

12

pH

Figure 5.25 The upper and lower potential limits of marmatite flotation with butyl xanthate as a collector in the presence of GX2 Table 5.1 Potential area of flotation of zinc-iron sulphide with butyl xanthate as a collector in the presence of GX2 Minerals

Marmatite

Arsenopyrite

Pyrrhotite

pH

Pulp potential Eh(mV, SHE)

4.5

6.5

9.2

Eh(u)

> 800

560

520

Eh(l)

290

360

480

Eh(u) Eh(l)

Eh(u) Eh(l)

No floatable

No floatable

Similarly, the influence of pulp potential on the flotation of marmatite, arsenopyrite and pyrrhotite with 10-4 mol/L butyl xanthate as a collector in the presence of 120 mg/L (l-carbonic sodium-2-acetaic sodium) propanic sodium dithio carbonic sodium (TX4) has been tested. The results are given in Fig. 5.26 and Table 5.2. It can be seen from Fig. 5.26 that at pH = 4.5 marmatite has wide floatable potential range from 0.3 V extended to above 0.7 V, at pH= 6.5 the floatable potential range is about 0.3 - 0.4 V, and at pH = 9.2 marmatite is not floatable. Table 5.2 demonstrates that in these conditions, arsenopyrite and 132

5.6

Role ofPolyhydroxyl and Poly Carboxylic Xanthate in the Flotation ofZinc-Iron Sulphide

pyrrhotite are not floatable. These results indicate that the flotation separation of marmatite from arsenopyrite and pyrrhotite may be accomplished using butyl xanthate as a collector and TX4 as a depressant in weak acidic pH media by modifying pulp potential. 100r---------~--------,

~

'6

80

BX:10-4mol/L

60

- - pH=4.5 .......... pH=6.5 --6- pH=9.2

Q)

> o ~

~

40

20 0L..----L-_----I_ _ 200 300 400

--1..-_ _L . . - - _ - . L - _ - - - - l

500

600

700

Eh(mV, SHE)

Figure 5.26 Flotation recovery of marmatite as a function of pulp potential with butyl xanthate as a collector in the presence ofTX4 Table 5.2 Potential area of flotation of zinc-iron sulphide with butyl xanthate as a collector in the presence ofTX4 Minerals

Marmatite

Arsenopyrite

Pyrrhotite

pH

Pulp potential Eh(mV, SHE)

4.5

6.5

Eh(u)

>800

490

Eh(l)

260

300

Eh(u) Eh(l)

Eh(u) Eh(l)

9.2 No floatable

No floatable

No floatable

5.6.3 Adsorption of Polyhydroxyl and Polycarboxylic Xanthate on Zinc-Iron Sulphide Figure 5.27 presents the adsorption isotherms of 2,3-dihydroxyl propyl dithiocarbonic sodium (GX2) on marmatite, arsenopyrite and pyrrhotite. The adsorption of GX2 on these three zinc-iron sulphides is increased with its 133

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

concentration until 100 mg/L, after which the adsorption reaches a plateau. The adsorption is increased in the order of arsenopyrite> pyrrhotite> marmatite, which is just the order of GX2 depression on these three minerals. The effect of pH on the adsorption GX2 on the three minerals is given in Fig. 5.28. It follows that the adsorption of GX2 increases with the increase of pH. The adsorption of GX2 increases rapidly at pH > 4 and reaches a plateau at pH> 6, which accounts for the depression action of GX2 on these three minerals in Fig. 5.21. 10 9

8

~ E

7

~ 6 (I)

..0

5 0 """ rf'J

"'0

ro

4

0

3

1: ;:j

E

pH=6.0 - - Marmatite+GX2 ....... Pyrrhotite+GX2 ........ Arsenopyrite+GX2

0

50

100 150 200 Cone. of GX2(mg/L)

250

Figure 5.27 Adsorption of GX2 on marmatite, arsenopyrite and pyrrhotite as a function of initial concentration of GX2 10 9 8

bi3 7 00 E 6

~ (I)

..0

5

rf'J

4

""" 0

~

1: ;:j 3 0 E 2

- - Marmatite+GX2 ....... Arsenopyrite+GX2 ........ Pyrrhotite+GX2

1 0

2

4

8

6

10

12

pH

Figure 5.28 Adsorption of GX2 on marmatite, arsenopyrite and pyrrhotite as a function of pH (GX2: 120 mg/L)

Figure 5.29 and Fig. 5.30, respectively, show the adsorption of (l-carbonic sodium-2-acetaic sodium) propanic sodium dithio carbonic sodium (TX4) on 134

5.6

Role ofPolyhydroxyl and Poly Carboxylic Xanthate in the Flotation ofZinc-Iron Sulphide

marmatite, arsenopyrite and pyrrhotite as a function of concentration and pll, It can be seen from Fig. 5.29 that with the increase of TX4 concentration, the adsorption of TX4 on the three minerals is increased. At a concentration above 60 mg/L, the adsorption reaches a plateau. Adsorption magnitude is in the order of arsenopyrite> pyrrhotite> marmatite, which explains the depression order of TX4 on the three minerals in Fig. 5.24. Figure 5.30 shows that the adsorption of TX4 on the three minerals increases with the increase of pH. At pH > 4, the adsorption of TX4 on these three minerals reaches the maximum, which accounts for the depressing action ofTX4 on the three minerals at pH > 6 in Fig. 5.23. The adsorption maximum is corresponding to the maximum depression. 4.0 r - - - - - - - - - - - - - - - - - - - , 3.5 ,@3.0 On E ~ 2.5 (1)

-€

~ 2.0 ] E 1.5 :: o E
- - Marmatite+TX4 ...... Pyrrhotite+TX4 --Ir- Arsenopyrite+TX4

0.5

o

20

40 60 80 Cone. of TX4 (mg/L)

100

120

Figure 5.29 Adsorption of TX4 on marmatite, arsenopyrite and pyrrhotite as a function of initial concentration of TX4 4.0..----------------. 3.5

'bO

~

3.0 2.5

.eo

2.0

]

1.5

~ rn

+J

s::

g E

1.0 0.5

- - Marmatite+TX4 ...... Pyrrhotite+TX4 --Ir- Arsenopyrite+TX4

0.0'----_--'--_---'--_----"'_ _-'--_--'------' 2 4 6 8 10 12 pH

Figure 5.30 Adsorption of TX4 on marmatite, arsenopyrite and pyrrhotite as a function of pH

135

Chapter 5 Roles of Depressants in Flotation of Sulphide Minerals

5.6.4

Effect of Polyhydroxyl and Polycarboxylic Xanthate on the Zeta Potential of Zinc-Iron Sulphide Minerals

Zeta potential of mannatite, arsenopyrite and pyrrhotite as a function of pH are presented in Fig. 5.31. It can be seen that these three minerals are negatively charged in wide pH range in water. No PZC are observed for arsenopyrite and pyrrhotite. In stronger acidic media, mannatite has a small positive zeta potential and the PZC occurs around pH= 5. The effects of polyhydroxyl and polycarboxylic xanthate on the zeta potential of the three minerals are given in Fig. 5.32 and Fig. 5.33 respectively. It can be seen from Fig. 5.32 that with the increase of GX2, the negative zeta potential of mannatite, pyrrhotite and arsenopyrite increase. The negative zeta potential reach the maximum and remained stable at the concentration of GX2 100 mg/L. The increasing magnitude of the zeta potential in the presence of GX2 is in the order of arsenopyrite> pyrrhotite> mannatite, which explains the adsorption order of GX2 on the three minerals. Figure 3.32 also suggests that the adsorption of anionic depressant 2,3-dihydroxyl propyl dithiocarbonic sodium (GX2) on negatively charged mannatite, arsenopyrite and pyrrhotite may be due to the chemical interaction. 10

o -Marmatite ..... Arsenopyrite ...... Pyrrhotite

:> -10 S

] -20 1: Q)

8. -30 ~

N

-40

-50 2

4

6

pH

8

10

12

Figure 5.31 Zeta potential of zinc-iron sulphide minerals as a function of pH

It can also be seen from Fig. 5.33 that with the increase of (l-carbonic sodium-2-acetaic sodium) propanic sodium dithio carbonic sodium (TX4), the negative zeta potential of mannatite, pyrrhotite and arsenopyrite increase. The negative zeta potential reach the maximum and remained stable at the concentration of TX2 60 mglL. The zeta potential in the presence of TX2 increases in the order of arsenopyrite> pyrrhotite> mannatite, which is corresponding to the adsorption order ofTX2 on the three minerals. Figure 3.33 also suggests that the adsorption of anionic depressant TX2 on negatively charged mannatite, arsenopyrite and pyrrhotite may be due to the chemical interaction. 136

5.6

Role ofPolyhydroxyl and Poly Carboxylic Xanthate in the Flotation ofZinc-Iron Sulphide -10....----------------, -15 -20 -25 __ -30

5> ]

C

-35 -40

E -45

o ~ -50

~ -55 -60 -65

pH=6.5 ....... Marmatite ___ Pyrrhotite -.- Arsenopyrite

- 70 L-..JL...--L-J----l----l.----.l.-..L~:=::L:~~==A....J o 25 50 75 100 125 150 Cone. of GX2 (mglL)

Figure 5.32 Zeta potential of zinc-iron sulphide minerals as a function of GX2 concentration

-60

pH=6.5 - - Marmatite ___ Pyrrhotite ....... Arsenopyrite

-70

>'

5 ]

-80

E

~ -90 0..

$3 Q)

N -100

-110

o

50

100

150

Cone. ofTX4 (mg/L)

Figure 5.33 Zeta potential of zinc-iron sulphide minerals as a function of TX4 concentration

5.6.5

Structure-Property Relation of Polyhydroxyl and Polycarboxylic Xanthate

The influence of three kinds of polyhydroxyl xanthate and four kinds of polycarboxylic xanthate on arsenopyrite and pyrrhotite flotation are further examined in order to compare the effect of the depressant structure on their depressing action. The formulae of three polyhydroxyl xanthates are shown in Table 5.3 and their depression on arsenopyrite flotation is given in Fig. 5.34. It is obvious that the depressing ability of polyhydroxyl xanthates is in the order of OX3 > OX2 > GXI. The more the hydroxyl groups in the depressant polyhydroxyl 137

Chapter5 Roles of Depressants in Flotationof Sulphide Minerals

xanthate, the stronger the depressing action. The depressing action increases with the increase of concentration of polyhydroxyl xanthates. At 120 mg/L, the flotation recovery can be below 30%. Table 5.4 shows the formulae of four polycarboxylic xanthates. Figure 5.35 demonstrates the results of the polycarboxylic xanthates on the depression of arsenopyrite. It can be seen that the depression of the polycarboxylic xanthates on arsenopyrite increases with increase of depressant concentration and is in the order of TX4> TX3 > TX2> TX1. At 100 mg/L, the flotation recovery can be below 20%.The more the carboxylic groups in the polycarboxylic xanthates, the stronger the depression on arsenopyrite. Table 5.3 Poly-hydroxyl Xanthates 2-hydroxyl ethyl dithio carbonicsodium

GX1

MW Formulas (HO)C2H4OCSSNa 160

2,3 dihydroxyl propyl dithio carbonic sodium

GX2

(HO)2C3HsOCSSN

190

aX3

(HO)sC6HgOCSSN

280

Compounds

Symbols

2,3,4,5,6, five hydroxylheptyl dithio carbonicsodium

Table 5.4 Poly-carboxylic xanthates Compounds Sodiumacetatedithio carbonic sodium Sodiumpropronate dithio carbonicsodium (l-carbonic sodium-2-hydroxyl) Sodium propronate dithio carbonicsodium (l-carbonic sodium-2-sodium acetate)Sodium propronate dithio carbonicsodium

Formulas MW Symbols TX1 (NaOOC)CH2OCSSNa 196 210 TX2 (NaOOC)C2RtOCSSNa (NaOOC)(HO)CHCH 292 TX3 (COONa)OCSSNa (NaOOCCH2)2(NaOOC) 328 TX4 COCSSNa

100.--------------------,

80 pH=6.0 --OX1 ...... OX2 '-""OX3

~ 60

'6 (1)

> o

g 40

~

20

o

20

40 60 80 100 120 140 160 180 Concentration (mg/L)

Figure 5.34 Flotation recovery of arsenopyrite as a function of polyhydroxyl xanthateconcentration with butyl xanthateas a collector

138

5.6

Role of Polyhydroxyl and Poly Carboxylic Xanthate in the Flotation ofZinc-Iron Sulphide 100...------------------,

pH=6.0 -TX1 .......TX2 "-""TX4 ........ TX3

80

~ 60

'6 Q)

o>

C)

~ 40

: :

20

o

20

40 60 80 100 Concentration (mg/L)

120

140

160

Figure 5.35 Flotation recovery of arsenopyrite as a function of polycarboxylic xanthate concentration with butyl xanthate as a collector

The depressing property of the organic depressant mainly depends on its structure. Some quantitative criteria have been used to determine the depressing ability of the organic depressants (Wang, 1982, 1997). The characteristic index of an organic depressant is defined as follows: (5-19) or (5-20) where X g is the group electronegativity of the polar head, for - OH, X g = 3.9; for -COOH, X g = 4.1; for -OCSS(H), X g = 2.7; X H electronegativity of hydrogen; n chain length of nonpolar group; f/J = 0.8, K = 20.

The other quantitative criterion is hydrophilic-hydrophobic balance value defined as: HLB} =

L (hydrophilic group value) -L (hydrophobic group value) + 7

(5-21)

The i value and HLB value of polyhydroxyl and polycarboxylic xanthates are calculated and shown in Table 5.5. It is obvious that the polyhydroxyl xanthate with more hydroxyl groups and polycarboxylic xanthate with more carboxylic groups have greater i or HLB value. Taking the i or HLB values as x-axis and the flotation recovery of arsenopyrite or pyrrhotite at depressant concentration 139

Chapter 5 Roles of Depressantsin Flotationof SulphideMinerals

120 mg/L as y-axis, the flotation recovery of arsenopyrite and pyrrhotite as a function of i or HLB values is given, respectively in Fig. 5.36 to Fig. 5.39. They demonstrate that the depressing ability of polyhydroxyl or polycarboxylic xanthates is closely related.to their i or HLB values. With the increase of i value or HLB value of polyhydroxyl and polycarboxylic xanthates, the flotation recovery of pyrrhotite and arsenopyrite decreases. The greater the i or HLB value of the depressant, the stronger the depressing action of the depressants. In other words, when the mineral-philic group is given as xanthate radical, more hydroxylic groups or carboxylic groups in the hydrocarbon chain structure may produce stronger depressing actions. Table 5.5 i and HLB value Depressants

i

Formulas

i1

i2

HLB}

GX1

(HO)CH2CH2OCSSNa

2.25

22.0

26.25

GX2

(HO)2C3HsOCSSNa

2.85

24.44

27.68

aX3

(HO)sC 6HgOCSSNa

3.45

31.76

31.95

TX1

(NaOOC)CH2OCSSNa

5.45

23.56

43.93

TX2

(NaOOC)CH2CH2OCSSNa

2.725

22.76

43.45

TX3

(NaOOC)(HO)CHCH(COONa)OCSSNa

7.25

30.0

64.45

TX4

(NaOOCCH2)2(NaOOC)CCOCSSNa

15.45

31.56

83.08

40~----------------'

___ Arsenopyrite --e- Pyrrhotite

35

~ 30

'6 Q)

>

8 25

~

20 15 '--"---'---'---'---'---'---'----'----'----'--'---'---'---'---'---'---'---'----'----'

20 22

24

26

28

30

32

34

36

38 40

Figure 5.36 Flotation recovery of arsenopyrite and pyrrhotite as a function of i} value of polyhydroxylxanthates

140

5.6

Role of Polyhydroxyl and Poly Carboxylic Xanthate in the Flotation ofZinc-Iron Sulphide 40 __ Arsenopyrite

35

-+- Pyrrhotite

30 ~

'#-

'6 25 Q)

;;> 0

oQ)

20

~

15 10 5

2

0

4

8

6

10

12

14

16

18

i2

Figure5.37 Flotation recovery of arsenopyrite and pyrrhotite as a function of i2 value of polycarboxylic xanthates 40r-------------------. __ Arsenopyrite

35

~

C Q)

-+- Pyrrhotite

30

o> ~ 25

~

20

~-------•

26

27

28

29 30 31 32 HLB1 Figure5.38 Flotation recovery of arsenopyrite and pyrrhotite as a function of HLB 1 value of polyhydroxyl xanthates 40,......------------------, __ Arsenopyrite

35

-+- Pyrrhotite

30 ~

'#-

'6 25 Q)

oo>

~ 20

15 10 '-------'-_.....L------'~__'___....a..._____"'

40

50

60

_

__'___"""____"........- - - '

70

80 90 HLB1 Figure5.39 Flotation recovery of arsenopyrite and pyrrhotite as a function of HLB 1 value of polycarboxylic xanthates

141

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

Abstract Two systems are discussed in this chapter, which are copper activating zinc-iron system with and without depressants. Firstly, the system in the absence of depressants is discussed. And it is obtained that at a specific pH the activation for each mineral occurs in a certain range. Through the electrochemical methods and surface analysis the entity contributing to the activation can be identified which are usually copper sulphides and vary for different minerals. Secondly, the system with depressants is researched. And also the effects of pulp potential on the activation are discussed. The same conclusion can be obtained as the one from the former system. Furthermore, zeta potential are involved in the discussion of activation and the mechanism can be explained from the changes of zeta potential. Similarly, the activation mechanism of this system is also studied through solution chemistry, bonding of activator with mineral surfaces and surface analysis. Keywords activation; electrochemical mechanism; surface analysis; pulp potential; depressants

6.1

Electrochemical Mechanism of Copper Activating Sphalerite

Flotation recovery of sphalerite as a function of pH with butyl xanthate as a collector in the presence and absence of activator is presented in Fig. 6.1. Evidently, sphalerite exhibits good flotation in weak acidic media and the recovery decreases sharply with the increase of pH in the absence of activator. The flotation of sphalerite is activated in wide pH range in the presence of activators such as CUS04 or Pb(N03)2. The recovery of sphalerite can be above 90% at pH < 12 using copper ion as an activator and at pH < lOusing lead ion as an activator. The electrochemical mechanism of cupric sulphate activating sphalerite has been studied. The measured cyclic voltammogram curves in aqueous solution at different pH with sphalerite compound electrode are shown as solid lines in Fig. 6.1.

6.1

Electrochemical Mechanism of Copper Activating Sphalerite 100.-------------------.

80 ~

'$-

'6

60

Q)

>

0

o Q)

~

40

20

o

---eu

-o-Pb ....... No

2

4

8

6

10

12

14

pH

Figure 6.1 Flotation recovery of sphalerite as a function of pH with butyl xanthate (10- 4mol/L) as a collector (activator: 2 x 10- 4mol!L)

At pH = 6.0, the anode peak appearing in the first potential scan corresponds to the reaction: ZnS=Zn2+ + S + 2e-

(6-1)

EJ = 0.265{V) It may be derived that elemental sulphur is not the final product of surface oxidation of sphalerite, because the potential associated with the second anode peak corresponds to the thermodynamic potential of the reaction (3-11a) (taking potential barrier of SO~- into consideration). The first cathode peak appearing in inverse scan is corresponding to the reaction in which SO~- is reduced to element S again. The second cathode peak with a lower potential contains two reactions. ZnS+2H+ +2e- =Zn+H2S

EJ =

-

(6-2)

0.886{V)

Zn 2+ + 2e- =Zn

EJ =

-

(6-3)

0.760{V)

In the second forward scan, at the negative potential, there appear two new anode peaks which correspond to the two inverse reactions above. At pH = 9.5, the surface oxidation reaction of sphalerite is given by: ZnS + 2H 20=Zn{OH)2 + S + 2H++ 2e-

(6-4)

EJ = O.665{V) 143

Chapter 6 Electrochemistry of Activation Flotation of Sulphide Minerals

The second anode peak at the higher potential still corresponds to the reaction in which the element S is oxidized to SO~- . A new cathode peak appears in the inverse scan, which may correspond to the inverse reaction of reaction (6-4). It can probably be said that the Zn(OH)2, which can not be moved away completely from the electrode surface, reacts with reduced element S to form ZnS. The third cathode peak at the lower potential represents the reaction in which the ZnS is reduced to Zn. So there appears a new anode peak in the second forward scan. At pH = 12.8, the anode current increases rapidly, which indicates that a more intense oxidation reaction occurs at the sphalerite surface. At the same time two anode peaks appearing in weak acid media and in weak alkaline media merge together, which shows that the surface oxidation reaction of sphalerite can be given by: ZnS + 6H20= ZnO;- + SO~- + 12H++ 8e-

(6-5)

EJ = 0.635(V) Because ZnO;- can not be completely dissolved on the anode surface, ZnO;can react with element S to form ZnS, so that the corresponding cathode peak appears in the inverse scan. The cyclic voltammogram for sphalerite compound electrode changes greatly in aqueous solution with the CUS04 concentration of 1.1 x 10- 4 mol/L, which is shown as dotted lines in Fig. 6.2. It can be seen that compared with the oxidation properties of sphalerite, the oxidation properties of those surface products change after sphalerite is activated by CUS04. In the term of initial oxidation potential, the surface products are superior to sphalerite in oxidation. At pH = 6.0, the preset peak (0.25 V) appearing in the anode scan perhaps corresponds to the following two reactions. CU2S + H 2S=2CuS + 2H++ 2e-

(6-6)

EJ=0.081(V) CuS + H2S=S - CuS + 2H+ + 2e-

(6-7)

EJ=0.148(V) The anode peak (0.4 V) is due to the reactions in which CU2S and S - CuS are oxidized further. (6-8) EJ = 0.408(V) S - CuS + 4H 20=CuS + SO~- + 8H++ 6e-

(6-9)

EJ = 0.357(V) When the SO~- potential barrier is considered, because the thermodynamic 144

6.1 Electrochemical Mechanism of CopperActivating Sphalerite

reversible potential of reaction (6-8) is relatively high, it can be regarded that CuS is not oxidized further at the upper limit of the scan potential shown in Fig. 6.1. In the inverse scan, there appear three cathode peaks from right to left, which sequentially correspond to the back reaction of reactions (6-8), (6-9) and the reduction reaction of element sulphur.

I

0.2 mA/cm2(pH=6.0, 9.5) 0.5 mA/cm2(pH=12.8)

Zn-r Zn2+

2

S-S04-/ ,/ ZnS- Zn2+,

2

Zn, H2S- ZnS

/A

S /-

)

pH=6.0

: '>

.:

......

----~

_/

,./

.> I I

--S-SO~-

ZnS- Zn(OHh, S .: pH=9.5 -::L »>

~

\

I

', /,/

~-

.>

/

[CuS04]=0 - - - [CuS04]=1X 10-4 mollL -1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

E(V, SHE)

Figure 6.2 Cyclic voltammetric curves for sphalerite compound electrode in buffer solution with different pH at 293K, KN03 concentration of 0.5 mol/L and sweep rate of20 mV/s

At pH= 9.5, the cyclic voltammogram of the sphalerite activated by CUS04 is relatively complex. At the upper limit of the scan potential shown in Fig. 6.1, a series of anode peaks appearing in forward scan sequentially correspond to reaction (6-9) and other reactions as follows: CU2S + HS-=2CuS + H+ + 2e-

EJ = -

0.126(V)

CuS+ HS-=S-CuS+ H++2e-

EJ = -

(6-11)

0.066(V)

CuS + 2H20=Cu(OH)2 + S + 2H+ + 2e-

EJ =

(6-10)

(6-12)

- 0.863(V)

145

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

It can be seen that CU(OH)2 may exist on the electrode surface as well as CU2S and CuS. The existence of element S depends on the oxidation potential. Anode peaks appearing in inverse scan are sequentially associated with reaction (6-9), the back reaction of reaction (6-12) and reduction reaction of element S (HS- is the reduction product). In high alkaline media (pH =12.8), where the upper limit of the scan potential is carried to 0.2 V, the anode peak is similar to the above and corresponds to the reaction that CU2S is oxidized to CuS and S - CuS. The anode peaks at the potential varying from 0.1 V to 0.2 V may contain reaction (6-12), reaction (6-7) and reaction (6-10). At that moment, the products at the electrode surface may comprise of CU2S, CuS, CU(OH)2 and element S, so that there appears a series of cathode peaks which indicate the corresponding back reactions. According to the analyses of the above cyclic voltammogramresults, it can not be determined that the active products at the sphalerite surface is CU2S or CuS. However this phenomenon may imply the essence of the process itself. It suggests that in the ZnS - CUS04 system the Cu concentration in active products keeps equilibrium with that in solution, which is strictly controlled by chemical potential. When the working potential is lower than the rest potential of CuS, the active products will absorb more Cu2+ ions from the solution to form high Cu/S ratio compound. 2+

-

CUxSx + yCu + 2ye

~

CUx + vS,

(6-13)

When the working potential is higher than the rest potential of CuS, sulphide will partly lose element Cu to form low Cu/S ratio compound. (6-14) In the actual flotation system, the activators on the sphalerite surface are some copper sulphide compounds containing different amounts of copper, namely some non-chemometric cupric sulphide. Based on the electrochemical theory of chalcocite in geologically minerogenic process, they may be chalcocite (CU2S), diulerite(Cu1.96S), CU1.71-1.82S, anilite(CU1.75S), geerite(CU1.60S), spionkopite (CUl.40S), Cul.12S and covellite (CuS).

6.2

Electrochemical Mechanism of Copper Activating Zinc-Iron Sulphide Minerals

6.2.1 Activation Flotation 4 The result of activation flotation of marmatite is given in Fig. 6.3 with 10- mol/L 146

6.2

Electrochemical Mechanism of Copper Activating Zinc-Iron Sulphide Minerals

4 ethyl xanthate as a collector and 2 x 10- mollL CUS04 as an activator. It follows that marmatite is floatable only at pH < 6 in the absence of cupric ion and the recovery is about 90%. At pH > 6, the recovery of marmatite decreases sharply to below 30%. However, marmatite exhibits very good flotation response at wider pH range in the presence of cupric ion showing the stronger activation of copper ion on marmatite. The recovery of marmatite increases to above 90% in the range ofpH=2-12. 100

2 2: CuS04' 2x 10-4 mol/L

80 .-...

'$.

'6

60

Q)

;;> 0

oQ)

~

40 20

o

1: No CUS04' 10-4 mol/L

2

4

6

pH

8

10

12

14

Figure 6.3 Flotation recovery of marmatite as a function of pH with ethyl xanthate (10- 4 mol/L) as a collector

6.2.2 Effect of Pulp Potential on Activation Flotation of Zinc-Iron Sulphide Minerals Figure 6.4 to Fig. 6.6 show the effect of pulp potential on the copper activation 4 flotation of marmatite, arsenopyrite and pyrrhotite in the presence of 10- mollL 4 2 Cu + with 10- mollL butyl xanthate as a collector. It is obvious that marmatite, arsenopyrite and pyrrhotite appear to have better flotation response at certain potential range at different pH conditions. Figure 6.4 shows that the initial potential of marmatite flotation is around 0.26 V and is almost independent of pH, but the upper limit potential of flotation varies with pH, which is higher at acidic pH value. The recovery of marmatite can be above 90% only at certain pulp potential ranges at given pH. The optimal flotation potential range is 0.35 - 0.6 V at pH = 4.5, 0.28 - 0.5 V at pH = 6.5 and 0.25 - 0.3 V at pH = 9.2. It shows the stronger activation of copper ion on marmatite flotation. Figure 6.5 demonstrates that arsenopyrite starts flotation at a potential around 0.18 V also independent of pH and ceases flotation at different potential values dependent on pH. The recovery is about 80% in the pH range 4.5 - 6.5 and 147

Chapter 6 Electrochemistry of Activation Flotation of Sulphide Minerals

potential range of 300 - 600 mY, indicating some activation of copper ion on arsenopyrite in weak acidic media. At pH = 9.2, the floatability of arsenopyrite is poor and the activation of copper ion is weak. 100 80 ".-.....

~

'6

60

(1)

;>

0

o(1)

40

~

____ pH=4.5 pH=6.5 -..- pH=9.2 -+-

20 0

200

300

400

500

600

Eh(mV, SHE)

Figure 6.4 Flotation recovery of marmatite as a function of pulp potential in the presence of 10- 4 mollL Cu2+ with 10- 4 mol/L butyl xanthate as a collector 100 r - - - - - - - - - - - - - - - - - - - ,

80

~ 60

'6 (1)

;>

o

g 40

~

20

BX: 10-4 mol/L CUS04: 10-4 mol/L ____ pH=4.5 -+- pH=6.5 -..- pH=9.2

OL..-....L--'----L...-~....L.__.L.._._L__'____L._____L__L.._.l...__.J.__'___L_--l-.....L........J

100 200

300 400 500 600 700 Eh(mV, SHE)

800

900

Figure 6.5 Flotation recovery of arsenopyrite as a function of pulp potential in the presence of 10-4 mol/L Cu2+ with 10- 4 mol/L butyl xanthate as a collector

The lower and upper limit flotation potential of pyrrhotite activated by copper ion change with pH as shown in Fig. 6.6. At pH = 4.5, pyrrhotite exhibits better flotation response with recovery about 80% in the potential range of 0.25 - 0.65 V indicating the stronger activation of copper ion. At pH= 6.5, the recovery of pyrrhotite reaches 80% around potential 0.3V. At pH = 9.2, the floatability of pyrrhotite is low. Taking the flotation recovery 50% as a criterion, above which the mineral is 148

6.2 Electrochemical Mechanism of Copper Activating Zinc-Iron Sulphide Minerals 100.-----------------, 80

~ 60

'6 OJ

o> ~

~

40 20

100

200

300

400

500

600

700

Eh(mV, SHE)

Figure 6.6 Flotation recovery of pyrrhotite as a function of pulp potential in the presence of 10-4 mol/L Cu2+ with 10-4 mol/L butyl xanthate as a collector

considered to be floatable or otherwise nonfloatable, the upper and lower potential limits of flotation of marmatite, arsenopyrite and pyrrhotite at different pH are presented in Table 6.1. It follows that in weak acidic media these minerals have wider potential area of activation flotation by copper ion, which is narrowed in alkaline media. Marmatite and arsenopyrite show larger upper limit potential and pyrrhotite exhibits smaller upper limit potential. Table 6.1 Eh-pH area of flotation of Zinc-Iron sulphide minerals in the presence of 10-4 mol/L Cu2+ with 10-4 mol/L butyl xanthate as a collector Minerals Marmatite Arsenopyrite Pyrrhotite

Upper and Lower Limit Potential Eh(m~ SHE) Eh(u) Eh(l) Eh(u) Eh(l) Eh(u) Eh(l)

pH

4.5 >800 260 800 180 >700 110

6.5 590 260 750 170 480 150

9.2 500 220 500 180 460 260

6.2.3 Electrochemical Mechanism of Copper Activating Marmatite Electrochemical mechanism of copper activating marmatite is investigated by using voltammetric method. The voltammogram of the marmatite electrode in the presence of 10-4 mol/L Cu2+ is presented in Fig. 6.7. It can be seen that in the presence of cupric ion marmatite surface exhibits the electrochemical character of activation products. In the light of Eh-pH diagram of the Cu-S-H20 system 149

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

(Woods et aI., 1987; Young et aI., 1998), the cathodic peaks CI, C2 in Fig. 6.7 are respectively corresponding to the reactions ofEqs. (6-15) and (6-16). C t : 2CuS+H+ +2e- ~Cu2S+HS-

(6-15) (6-16)

The anodic peaks are respectively corresponding to the reactions (6-10) to (6-12). It indicated that the surface product of the marmatite electrode in the presence of cupric ion is CuS. 10

o -10

~< -20 :::i

~

-30 -40 - 50

L.-......L.-L---l..-'-----1..---L-.....l.....-.I"---L---I---I....---I---L.---J.---I....---I---L.----I

-0.6 -0.4 -0.2 0.0

0.2 0.4 0.6

E(V)

0.8

1.0

Voltammogram of marmatite electrode activated by Cu2+ in 0.1 mol/L KN0 3 buffer solution of pH = 9.18 (scanning rate: 20 mV/s, from cathode to anode)

Figure 6.7

The activating mechanism of cupric ion on marmatite is further investigated by holding marmatite electrode for 2 min at different potential and then scanning from cathode to anode or from anode to cathode. The results are shown in Fig. 6.8. It can be seen from Fig. 6.8 that the less the holding potential is, the less the current of cathodic peak and the greater the current of anodic peak are. At - 278 mV, the cathode shows no sign of the reduction of CuS to CU2S. With the increase of the holding potential, the cathodic peak of reduction of CuS to CU2S appears. When scanning from anode to cathode as seen from Fig. 6.8(b), the current of anode and cathode peaks is higher at the holding potential -78 mV than at + 122 mV and +322 mV The formation of CuS is prominent. It suggests that the surface product of marmatite electrode in the presence of Cu2+ is CuS at higher potential and CU2S at lower potential.

6.2.4 Surface Analysis of Mechanism of Copper Activating Marmatite The FTIR spectra of ethyl xanthate adsorption on marmatite in the presence of 150

6.2

Electrochemical Mechanism of Copper Activating Zinc-Iron Sulphide Minerals

100.-------------------,

100.-----------------,

50 50

o

1

,-.

E

-50

~

0

~

::l

~ -50

~-100

--....... ---

-150 -200 -250

-0.8

-0.4

0.0

0.4

-278mV -78mV +122mV +322mV 0.8

--78mV - - - +122mV ....... +322mV

-100

1.2

-150 -0.8

L.--.L...-....L.......L--l...-..L...-..L.--L...-L.--.L...-....L.......L--l...-..L...-..L.--l........JL.--.L...-....L.......L---l

-0.4

~~ (a)

0.0

0.4

0.8

1.2

~~ (b)

Figure 6.8 Voltammograms of marmatite electrode activated by Cu2+ in 0.1 mol/L KN0 3 borate buffer solution of pH = 9.18 by holding 2.. minutes at a defmite potential (scanning rate: 50 mV/s). (a) from cathode to anode; (b) from anode to cathode

cupric ion is illustrated in Fig. 6.9. The characteristic bands of xanthate at 1195, 1125,1035 cm- 1 is observed indicating the formation ofCuEX. At pH= 8.8 and pH= 12.1, the intensity of reflection peak is almost the same. The interaction of marmatite with ethyl xanthate in the presence of cupric ion can take place in wide pH region even in strong pH, which accounts for the activation action of copper ion on marmatite flotation in wide pH range as shown in Fig. 6.3.

1200

1000 Wave numbers (crrr")

800

Figure 6.9 FTIR spectra of marmatite in the presence of cupric ion and ethyl xanthate (CUS04: 5 x 10-3 mol/L; KEX: 5 x 10- 3 mollL; 1: pH = 8.8; 2: pH = 12.1) 151

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

The interaction of marmatite with xanthate in the presence of cupric ion is investigsted by using Auger Electron Spectroscopy sputtering at different time and the results are given in Fig. 6.10. In the presence of cupric ion and xanthate, the marmatite surface shows the presence of carbon and copper elements besides iron and zinc. At sputtering 0.4 min, i.e. the etching depth about 1 nm, the peak intensity of carbon and copper elements decreases greatly or disappears. It indicates that the interaction between cupric ion and/or xanthate and marmatite surface may take place mainly at surface depth about 1 nm. The substitution of copper ion on zinc ion results in the formation of CuS or CU2S film on marmatite surface, which reacts with xanthate to form hydrophobic cuprous xanthate. 40 . - - - - - - - - - - - - - - - - - - - - ,

--+- C ~Fe

30

--..- Cu ~Zn

10

o

0.5

1.0 1.5 2.0 Etch depth (run)

2.5

3.0

Figure 6.10 Depth profile of Auger Electron Spectroscopy (AES) sputtering of marmatite surface in the presence of cupric ion and ethyl xanthate

6.3

6.3.1

Activation of Copper Ion on Flotation of Zinc-Iron Sulphide Minerals in the Presence of Depressants Effect of Depressant on the CUS04 Activating Flotation of Zinc-Iron Sulphide Minerals

The influence of copper ion on the flotation of zinc-iron sulphide minerals in the presence of depressant with butyl xanthate 1.0 x 10- 4 mol/L as a collector is presented in Fig. 6.11 to Fig. 6.14. It can be seen from Fig. 6.11 and Fig. 6.12 that in the presence of 120 mg/L 2-hydroxyl ethyl dithio carbonic sodium (GXl) and 2,3 dihydroxyl propyl dithio carbonic sodium (GX2), marmatite is activated by copper ion and exhibits very good flotation with a recovery above 90% in the pH range of 4 - 8. The flotation of arsenopyrite and pyrrhotite is poor with a 152

6.3

Activation of Copper Ion on Flotation of Zinc-Iron Sulphide Minerals in the ... 100r-------------------,

•

•

•

80

•

•

CUS04: 10-4mol/L

BX: 10-4mol/L

~ 60

'6 > § 40 Q)

~

20

o

___ Marmatite --.- Arsenopyrite --6- Pyrrhotite 2

4

6

pH

8

10

12

14

Figure 6.11 Flotation response of zinc-iron sulphide minerals as a function of pH in the presence of 120 mg/L OX1 and LOx 10- 4mollL CUS04 with butyl xanthate as a collector 100

~

80

CUS04: 10-4mol/L

,.-.

BX: 10-4mol/L

'$. 60

'6 Q)

>

0

oQ)

40

~

20 ___ Marmatite --.- Arsenopyrite --6- Pyrrhotite 0

2

4

6

pH

8

10

12

14

Figure 6.12 Flotation response of zinc-iron sulphide minerals as a function of pH in the presence of 120 mg/L OX2 and 1.0 x 10- 4mollL CUS04 with butyl xanthate as a collector

recovery of below 50% at pH > 6. It indicates that the GXl and GX2 have stronger depressing action on xanthate flotation of pyrrhotite and arsenopyrite even in the presence of copper ion and does not affect the flotation of marmatite, showing the possibility of flotation separation of marmatite from arsenopyrite and pyrrhotite in the presence of copper ion and depressant GXl or GX2. Figure 6.13 and Fig. 6.14 demonstrate the flotation results of zinc-iron sulphide minerals with 1.0x 10-4 mol/L butyl xanthate as a collector in the presence of (I-carbonic sodium-2-hydroxyl) sodium propronate dithio carbonic sodium (TX3) or (l-carbonic sodium-2-sodium acetate) sodium propronate dithio carbonic 153

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals 100..----------------.

80 CUS04: 10-4 mol/L

~

'6

BX: 10-4 mol/L

60

Q)

> o

g

~

40

20

-+- Marmatite -+- Arsenopyrite

.-.- Pyrrhotite

o

2

4

6

pH

8

10

12

14

Figure 6.13 Flotation response of zinc-iron sulphide minerals as a function of pH in the presence of 120 mg/L TX3 and 1.0 x 10- 4mol/L CUS04 with butyl xanthate as a collector

100 80 CUS04: 10- 4 mol/L BX: 10- 4 mol/L

".-...,

~

'6

60

Q)

>

0

oQ) cG

40

20

--~: :

-+- Arsenopyrite --6-

0

• •

Pyrrhotite

2

4

6

pH

8

10

12

14

Figure 6.14 Flotation response of zinc-iron sulphide minerals as a function of pH in the presence of 120 mg/L TX4 and 1.0 x 10- 4 mol/L CUS04 with butyl xanthate as a collector

sodium (TX4) and 1.0 x l 0- 4mol/L CUS04. It is obvious that the flotation of marmatite activated by copper ion is not affected by the addition of depressants TX3 and TX4 and the recovery is above 80% in the pH region of 2 - 10. However, TX3 and TX4 exhibit strong depressing action on arsenopyrite and pyrrhotite even in the presence of CUS04 and the flotation recovery of these two minerals are below 30% at pH> 6. It demonstrates that the flotation separation of marmatite from arsenopyrite and pyrrhotite may be completed in the presence of the depressant TX3 or TX4 and CUS04 with butyl xanthate as a collector. 154

6.3

Activation of Copper Ion on Flotation of Zinc-Iron Sulphide Minerals in the ...

6.3.2 Influence of Pulp Potential on the Copper Ion Activating Flotation of Zinc-Iron Sulphide Minerals in the Presence of Depressant Figure 6.15 to Fig. 6.18 show the influence of pulp potential on the flotation of zinc-iron sulphide minerals in the presence of depressant and CUS04 with butyl xanthate as a collector. It can be seen from Fig. 6.15 that in the presence of depressant 120 mg/L GX2 copper ion still activates the xanthate flotation of marmatite. The initial potential of marmatite flotation is around 0.3 V and the recovery can reach to 90%. The upper limit potential of marmatite flotation can be high to 0.8 V at pH= 4.5 and around 0.6 V at pH= 6.5 or 9.2. Figure 6.16 shows that the flotation of arsenopyrite is very poor at various pH and wide potential region. The recovery is below 20% in most potential ranges and the maximum recovery is 35%. It further indicates the possibility of flotation separation of mannatite from arsenopyrite and pyrrhotite using xanthate as a collector in the presence of copper ion and depressant GXl or GX2 by modifying the pulp potential. Similarly, in the presence of 150 mg/L TX4, copper ion still activates the xanthate flotation of marmatite as shown in Fig. 6.17. Marmatite starts flotation at a potential around 0.3 V with recovery of above 80%. At pH = 4.5, the upper limit potential of flotation of marmatite can be high to above 0.6 V with a recovery about 90%. At pH= 6.5 and 9.2, the upper limit potential of flotation of marmatite decreases to about 0.5 V. Arsenopyrite can not be floated in the same conditions with a recovery of below 30% as seen from Fig. 6.18. This result also suggests that the flotation separation of marmatite from arsenopyrite may be accomplished by using TX4 as a depressant and xanthate as a collector in the presence of copper ion through the control of pulp potential and pH. 100,..-------------------,

80

BX:10-4mol/L CUS04: 10-4mol/L

____ pH=4.5 pH=6.5 ......... pH=9.2 --+-

20

200

300

400

500

600

700

Eh(mV, SHE)

Figure 6.15 Flotation recovery of marmatite as a function of pulp potential in the presence of 120 mg/L GX2 and 10- 4 mol/L CUS04

155

Chapter 6 Electrochemistry of Activation Flotation of Sulphide Minerals 100. - - - - - - - - - - - - - - - - - - - - - , 80

~

C

BX: 10-4 mol/L CUS04: 10-4mol/L __ pH=4.5 pH=6.5 --6- pH=9.2

60

-+-

~

o>

~ 40

200

300

400

500

600

700

800

900

Eh(mV, SHE)

Figure 6.16 Flotation recovery of arsenopyrite as a function of pulp potential in thepresence of 120mg/LGX2and 10- 4 mollL CUS04 100. . . . . - - - - - - - - - - - - - - - - - ,

80

~

60

C ~

oo>

BX: 10-4 mol/L CUS04: 10-4 mol/L

~ 40

____ pH=4.5 pH=6.5 --6- pH=9.2

-+-

20

200

300

400

500

600

Eh(mV, SHE)

Figure 6.17 Flotation recovery of marmatite as a function of pulp potential in the presence ofTX4 and CUS04 ]00....-------------------,

80 BX:10-4mol/L CUS04: 10-4 mol/L

~ 60

'6

__ pH=4.5 pH=6.5 --6- pH=9.2

il)

o>

-+-

~ 40

200 300

400

500 600

700 800

900

Eh(mV, SHE)

Figure 6.18 Flotation recovery of arsenopyrite as a function of pulp potential in thepresence ofTX4 and CUS04

156

6.3

Activation of Copper Ion on Flotation of Zinc-Iron Sulphide Minerals in the ...

6.3.3 Zeta Potential of Zinc-Iron Sulphide Minerals in the Presence of Flotation Reagents The zeta potential of zinc-iron sulphide minerals in various flotation reagents are presented in Fig. 6.19 to Fig. 6.22 as a function of pH. It can be seen from Figs. 6.19 and 6.20 that marmatite and arsenopyrite are negatively charged in wide pH range. By addition of 1.0 x 10- 4 mol/L butyl xanthate, the zeta potential of marmatite and arsenopyrite are more negative indicating the adsorption of xanthate on these two minerals. Depressants 2,3-dihydroxyl propyl dithio carbonic sodium (GX2) evidently affect the zeta potential of marmatite and arsenopyrite, which became much more negative in the presence of 120 mg/L GX2, indicating the adsorption of GX2 on these two minerals. The negative zeta potential of arsenopyrite is increased more than that of marmatite in the presence of GX2, o -10

>' 5 -20

3

E -30

"0 ~ -40

___ Marmatite --.... Marmatite+BX ......... Marmatite+GX2 -.- Marmatite+GX2+BX+CuS04

~

N

-50

-60

6

4

2

pH

8

12

10

Figure 6.19 Zeta potential of marmatite as a function of pH in butyl xanthate/GX2/ CUS04 system ___ Arsenopyrite --.... Arsenopyrite+BX ......... Arsenopyrite+GX2 ......... Arsenopyrite+GX2+BX+CuS04

0 -10

>' -20 5

] -30

= 8. -40 Q)

~ Q)

N

-50

-60 -70

2

4

6

pH

8

10

12

Figure 6.20 Zeta potential of arsenopyrite as a function of pH in butyl xanthate/ GX2 /CUS04 system 157

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

suggesting the stronger adsorption of GX2 on arsenopyrite than on marmatite. Further by the addition of 1.0x 10-4mol/L CUS04, the zeta potential of marmatite and arsenopyrite moves towards positive and becomes less negative compared with the absence of CUS04, showing the adsorption of copper ion on these two minerals. The change of zeta potential in the presence of CUS04, is more for mannatite than arsenopyrite. It demonstrates that in butyl xanthate/depressant/ CUS04system copper ion may be adsorbed more on marmatite and the GX2 may be adsorbed more on arsenopyrite accounting for the flotation results from Fig. 15 to Fig. 16, i.e. the depressants GX2 exhibits stronger depressing action on arsenopyrite even in the presence of copper ion and affect little on the copper ion activation flotation of marmatite. Similarly, Figs. 6.21 and 6.22 show that by the addition of 1.0x 10-4mollL butyl xanthate, the zeta potential of marmatite and arsenopyrite are more negative. The addition of depressants (I-carbonic sodium-2-sodium acetate) sodium propronate dithio carbonic sodium (TX4) evidently affects the zeta potential of marmatite and arsenopyrite, which becomes much more negative in the presence of 150 mg/L TX4, indicating the adsorption of TX4 on these two minerals. TX4 makes the zeta potential of arsenopyrite more negative than that of marmatite, suggesting the stronger adsorption of TX4 on arsenopyrite than on marmatite. In the presence of 1.0x 10- 4mol/L CUS04, the zeta potential of marmatite and arsenopyrite moves towards positive and becomes less negative compared with the absence of CUS04, showing the adsorption of copper ion on these two minerals. The change of zeta potential in the presence of CUS04, is more for marmatite than arsenopyrite. It demonstrates that in the butyl xanthate/depressant/Cubfi, system copper ion may be adsorbed more on marmatite and TX4 may be adsorbed more on arsenopyrite accounting for the flotation results that the TX4 exhibites o -10

-20

>' -30

~Marmatite

Marmatite+BX .......... Marmatite+TX4 -.- Marmatite+TX4+BX -+-

+CuS04

~ -40

.s

~ -50 '0 ~-60

~

-70

-80

-90 -1 00 L-----'----'-----'_-L...----"--~_ 8 4 2 6 pH

_L.._~----'-____'

10

12

Figure 6.21 Zeta potential of marmatite as a function of pH in butyl xanthate/ TX4 /CUS04 system

158

6.4 Surface Chemistry of Activation of Lime-Depressed Pyrite

o

____ Arsenopyrite ......- Arsenopyrite+BX ......- Arsenopyrite+TX4 -.- Arsenopyrite+ TX4+BX

-10 -20

-30

>' ;gE

-50

~

-70

"E

+CUS04

-40 -60

~ -80 ~ -90

-100 -110 -120 -130

L...--.-L..---J-----'-_.L.-.---L--.L-----L_-'----L-----'

2

4

6

pH

8

10

12

Figure 6.22 Zeta potential of arsenopyrite as a function of pH in butyl xanthate/ TX4/CuS04 system

stronger depressing action on arsenopyrite even in the presence of copper ion and does not influence the copper ion activation flotation of marmatite.

6.4

Surface Chemistry of Activation of Lime-Depressed Pyrite

In the flotation of poly-metallic sulphide minerals, especially complex sulphide ores, the pyrite can only be depressed at very high additions of lime CaO (at pH> 12) such is the case at the Fankou Mine in China where the pyrite is very floatable. After the flotation of galena and sphalerite, the activation and flotation of pyrite caused a predicament due to the extremely high pH. Sulfuric acid is the main activator for the flotation of lime-depressed pyrite, which causes environmental and calcification problems due to the large additions of sulfuric acid required. As suggested from Fig. 5.5 the formation of Ca(OH)2 and CaS04 takes place at the pyrite surface because of the high pH and the oxidizing atmosphere. At high pH, the formation of Fe(OH)3 is also expected .These hydrophilic species account for the complete depression of pyrite by lime treatment. The activation flotation of the lime-depressed pyrite is to remove these hydrophilic coatings.

6.4.1

Activation Flotation of Lime-Depressed Pyrite

A series of organic acids, inorganic acids and their salts have been tested as possible activators for the flotation of lime depressed pyrite. The results are presented in Fig. 6.23 and Fig. 6.24. It can be seen from Fig. 6.23 that oxalic acid is the strongest activator in the organic acid series. When the concentration of 159

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

oxalic acid is greater than 5 x 10- 3 mol/L, the flotation of lime depressed pyrite can be restored and reaches 94%. The activating efficiency descends in the order of oxalic acid> butyl bicarboxylic acid> propanic acid> acetic acid> poly amino phosphoric acid. For the inorganic acid series, it follows from Fig. 6.24 that sulfuric acid is the strongest activator. The activation flotation of lime depressed pyrite can reach 80% with 5 x 10-3 mol/L. The activating efficiency decreases in the order of sulfuric acid> poly-phosphoric acid> phosphoric acid> carbonic acid> hydrochloric acid. At a high dosage, phosphoric acid and carbonic acid also exhibit good activation ability. According to the results above, the activating efficiency of oxalic acid and sulphuric acid on the lime depressed pyrite ore (flotation tailing 100..------------------, 90 ~

'$.

~ Q)

6o

~

80 70 60 50 40 30 20 10

o

- - H 2C 204 ~ C4H 8(COOHh ........... C3H6COOH -6-CH3COOH ~ N(CH 2P0 3H h

15

5 10 Concentration (l0-3 mol/L)

Figure 6.23 Flotation recovery of lime-depressed pyrite activated by organic acids vs. concentration of activators 100 90 80 70 ~

'$.

60

Q)

50

'6 ;> 0

oQ)

~

40

--H2SO4

~(HP04)x

30

........... H3P04

20

~HCl

~H2C03

10

o

5

10

15

Concentration (l0-3 mol/L)

Figure 6.24 Flotation recovery of lime-depressed pyrite activated by inorganic acids vs. concentration of activators

160

6.4

Surface Chemistry of Activation of Lime-Depressed Pyrite

from the Fankou Pb-Zn circuit) has been tested. The results are compared in Table 6.2. It can be seen that both oxalic acid and sulphuric acid can be used as effective activators for the flotation of lime-depressed pyrite. A pyrite concentrate with a high sulphur grade and recovery can be obtained. Table 6.2 also shows that,

however, the dosage of oxalic acid is half that of sulphuric acid, which suggests that organic acids compete with sulphuric acid and reduce the environmental impact as well as calcification of pipelines. Table 6.2 The results of activation flotation of lime-depressed pyrite ore from the tailing of flotation of Pb-Zn sulphide ores Experiments Oxalic acid 8 kg/t Ethyl xanthate 220 g/t, pine oil 130 g/t, pH = 7.8 Sulphuric acid 16 kg/t, ethyl xanthate 220 g/t, pine oil 130 g/t, pH = 7.2

Yield (%)

Grade (%)

Recovery (%)

S cone.

Products

52.23

48.92

94.84

S tailing

47.77

2.91

5.16

S feed ore

100.00

26.94

100.00

S cone.

46.88

50.67

93.74

S tailing

53.12

3.00

6.26

S feed ore

100.00

25.34

100.00

6.4.2 Solution Chemistry Studies on Activation Flotation of Lime-Depressed Pyrite Under certain conditions, oxalic acid (let L2- represent the oxalate anion) can form soluble complexes with calcium and iron ions. Fe 3+ + L2-=FeL+

(6-17)

K 1 = 107 .56 Fe 3+ +2L2-=FeL;

(6-18)

K2 = 1013.64 Fe 3+ +3L2-=FeV3

(6-19)

K3 = 1018.49 Me 2+ +L2-=FeL PI: Ca2+ 101.6;

Me 2+ + 2L 2- =

(6-20)

Fe2+ 103 .05 FeL22-

(6-21)

161

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

Let [Fe3+] express the total concentration of species containing Fe 3+, then (6-22) Let (/)0, (/)1, (/)2, (/)3, represent the percentage of various complexing species Fe 3+, FeL+, FeL; and Fer:3- , then [Fe 3+] 1 - -----------2-]+K 2 2 3 - [Fe +]t -1+K1[L - ]2 +K - ]3 2[L 3[L

(6-23)

K 1[L2- ]
(6-24)

(6-25)

(6-26)

(/)0 -

(/)1 =

If CL represents the total concentration of oxalate anion, then (6-27) (6-28) where a L is the coefficient for the protonation of oxalate. The percentage distribution of various complexes in such systems can be calculated using the equations above and the results are presented in Fig. 6.25. It may be seen that in the presence of oxalic acid, the dominant ferric complex is Fer:3- , even at lower dosage. The dominant complexes of Fe 2+ are FeL and FeL;-, 100

80

20

o-=-_---="'-----=_ _

- - L ._ _- - ' - - _ - - - - '_ _---'

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

19c

Figure 6.25 The percentage distribution of complexing species formed between oxalate and metal ions vs. the concentration of oxalate

162

6.4

Surface Chemistry of Activation of Lime-Depressed Pyrite

respectively, at a concentration of oxalate greater than 10-3 mol/L and 10-2 mol/L. The formation of CaL occurs only at a concentration of oxalate greater than 10-2 mol/L. Similarly, for the system of iron/calcium/phosphate, the percentage distribution of various complexes can also be calculated using solution equilibrium calculations as shown in Fig. 6.26. It follows that depending on solution pH, the dominant complexes is CaPO~ at pH= 10, whereas CaHP04(aq) and CaH2PO; are dominant at pH= 8. The soluble complexes formed by activators will desorb cation from the lime depressed pyrite surface, which will expose a fresh pyrite surface and activate pyrite flotation. Therefore, the moderately strong acids such as oxalic acid and phosphoric acid exhibit a strong activation action on lime-depressed pyrite because of their ability to decrease pulp pH and to form soluble complexes with hydrated surface cations. 100 90

Ca2+ /

80 70 ~

?f(

60

0

50

=.~

.0

·5 40 rJ)

0 30

/ ,- - - - - - - CaP04"

/

,

20 10

/

/

Calcium/phosphate system

/

CaHP04(aq)

Solid line pH=8 Dashedline pH= 10

,,

' Ca2+

"

O-=:::;;;;~~

-4.0

/

-3.5

__ -3.0

L..-----';~ _ _L..--~--'--_-----I

-2.5

-2.0

-1.5

-1.0

19c

Figure 6.26 The percentage distribution of complexing species formed between phosphate and calcium ions vs. the concentration of phosphate

6.4.3 The Bonding of the Activator Polar Group with Surface Cation In order to form stable complexes with calcium 'and iron ions, the activator must have a strong affinity for the cations. The bonding strength can be determined by the group electronegativity as given by the following equation: n* +

1)

X g =0.31 ( -r- +0.5

(6-29)

163

Chapter 6

Electrochemistry of Activation Flotation of Sulphide Minerals

(6-30) where n* is the effective valence electron numbers; r is the covalent radius of the bonding atom; N is the valence electron numbers of bonding; P is the electron numbers of bonding atom bonded by neighboring atom B;X is electronegativity of element; i is the order number of the group; m is the number of electron pairs for the bond between A and B; s is the number of electron pairs unbonded between A and B; a, is the separated constant (usually taken as 2.7). The X g value reflects the bonding ability of the activator with a particular cation. A better activator should have a greater X g values for some activators which are calculated and given in Table 6.3. Also shown in Table 6.3 are the pKa values for the activators of interest. It can be seen from Table 6.3 that the activating ability is increased in the order of increase of X g values, and of decrease of pKa values. Table 6.3 index Ai

The pKa, Xg values and calcium salts of activators and their activity

Activator

Xg

pKa (20)

Ai=Xg/pKa

H2C204

3.74

1.27

2.94

H2SO4

3.69

1.9*

1.94

(HP0 4)n

3.6

1.94

1.86

H3P04

3.59

2.12

1.69

H2C03

3.62

6.39

0.57

HCI

2.69

Therefore, the efficiency of an activator depends on its acidity which decreases pulp pH, and its bonding ability with cations which forms soluble stable complexes with cations at the pyrite surface. In other words, a better activator should have a lower pKa value(stronger acidity) and a greater X g value (stronger affinity). A combination of these factors defines the activity index Ai as follows: (6-31) The greater the Ai value, the stronger the activating action of an activator. Table 6.3 demonstrates that the activating efficiency of activators is in the same order as 164

6.4

Surface Chemistry of Activation of Lime-Depressed Pyrite

the Ai value.

6.4.4 Surface Analysis of Lime-Depressed Pyrite in the Presence of Activator Figure 6.27 shows the XPS spectra of the pyrite surface under different conditions. The characteristic peaks Ca(2p),Ca(2s) and Fe3+ disappear by the addition of oxalic acid when compared to spectra in the absence of activator. In the XPS extended spectra for oxygen and carbon (presenting since the system is open to atmosphere) at the pyrite surface, no change is found after the addition of oxalic acid, indicating that no adsorption of oxalate occurs at the pyrite surface. The results in Fig, 6.27 also indicate that the soluble complexes formed by activators with cation hydroxides ofCa(OH)2 and Fe(OH)3desorb from the pyrite surface.

CaO: 320 mg/L H2C20 4 : 10-3 mallL

CaO 320 mg.l!

o Figure 6.27 activator

200

400 Binding energy(eV)

600

800

XPS spectra of lime depressed pyrite in the absence and presence of

Figure 6.28 presents the anodic current of the pyrite electrode in different solutions. The results show that in the absence of activator, the anodic current evidently increases at a potential above 0.1 V and the strong oxidation of pyrite surface takes place at a high dosage of CaO. In the presence of activator, the potential at which the anodic current begins to rise is greatly increased, in the order· of hydrochloric acid, sulfuric acid, oxalic acid and phosphoric acid, indicating that activators will inhibit the surface oxidation of pyrite by the high dosage of CaO. Therefore, the mechanism that the organic or inorganic acid activates the lime-depressed pyrite flotation may be attributed to that these activators prevent the pyrite surface oxidation and expose the bared surface to recover pyrite floatability through their complexing action. 165

Chapter 6 Electrochemistry of Activation Flotation of Sulphide Minerals 60r-------------------------, 50

Cao: 320 mg/L Scan rate: 20 mV/s

No activator

HCI

"-'

E

~ 20 ::::s

U

10

o

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential(V, SHE)

Figure 6.28 Theanodic current of pyrite electrode in different solutions

166

Chapter 7 Corrosive Electrochemistry of OxidationReduction of Sulphide Minerals

Abstract The flotation mechanism is discussed in the terms of corrosive electrochemistry in this chapter. In corrosion the disolution of minerals is called self-corrosion. And the reaction between reagents and minerals is treated as inhibition of corrosion. The stronger the ability of inhibiting the corrosion of minerals, the stronger the reagents react with minerals. The two major tools implied in the research of electrochemical corrosion are polarization curves and EIS (electrochemistry impedance spectrum). With these tools, pyrite, galena and sphalerite are discussed under different conditions respectively, including interactions between collector with them and the difference of oxidation of minerals in NaOH solution and in lime. And the results obtained from this research are in accordance with those from other conventional research. With this research some new information can be obtained while it is impossible for other methods. Keywords corrosive electrochemistry; corrosive potential; corrosion inhibition; polarization curves; Electrochemistry Impedance Spectrum

7.1 Corrosive Electrochemistry As is known to all, the flotation mechanism of sulphide minerals can be explained based on electrochemistry because sulphide minerals have the semiconductor character and a series of electrochemistry reaction occurring in solution. After these reactions, the surface of sulphide minerals changes and forms a new phase. We called it as self-corrosion of sulphide minerals. As before, the essence of the reaction between the collector and the minerals is the formation of the hydrophobic entity on the mineral surface, and then minerals can be floated. We can find that the reaction between the collector and the minerals is similar to the depression on mineral self-corrosion. In the corrosion, we called this effect as inhibition, and this kind of reagent is an inhibiting reagent. There are many studies on corrosion, especially its research method and theory. Thus, we can get some new information on the mechanism of sulphide flotation from corrosive electrochemistry.

Chapter 7

7.1.1

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

Concept and Significance of Mixed Potential, Corrosive Potential and Static Potential

Corrosion can be divided into two parts, chemical corrosion and electrochemical corrosion. Chemical corrosion is a direct reaction between the medium and the materials, while electrochemical corrosion is an electron transformation on different surfaces of the metal because of conductivity. In this chapter, all corrosions are referred as electrochemical corrosions. There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, and and the

E:

E! ,

mineral electrode potential is E, the relationship among them is as follows: E = Ef/Ja + 1]l=e Ef/J

-1]2

(7-1)

where 'h and 172 are the over-potential for the anodic reaction and cathodic reaction, respectively. Mineral electrode potential is unequal potential, not only for the anode but also for the coupling cathode. Electrode potential E is named as the mixed potential of the whole electrode reactions. It is also named as the corrosion potential in corrosion or static potential for mineral in flotation electrochemistry. It can react only when E, - E, > O. In other words, sulphide minerals are corrupted only when the reduction equilibrium potential for an oxidant is higher than the oxidation equilibrium potential in sulphide mineral solution. Generally speaking, the oxidant is oxygen. Oxygen is the essential condition for the electrochemistry flotation of sulphide mineral. The concept of mixed potential can be extended to multi-electrode reactions on one electrode. If N electrode reactions happen on a single electrode simultaneously, the outside electrode current is up to zero. When N > 2, all these electrode reactions constitute the multi-electrode reaction coupled system, in which a part of the electrode reactions belong to the anodic reaction and the others are cathodic reaction. Given that the current of the anodic reaction is positive value and that of the cathodic reaction is negative value, the following formula can be given.

Ll N

1=

j

=0

(7-2)

j=l

I j is the current density of reaction j. Given that the over-potential of the anodic reaction is positive value and that of the cathodic reaction is negative value, and all N reactions happen at the same mixed potential, the following formula can be given. 168

7.1

Corrosive Electrochemistry

E=E¢J +n =E¢Je 2 +n =···=E¢Je +n./z =···=E¢Je N +n el •11 ' /2 '/ N j

(7-3)

•

Ee . is the equilibrium potential of reaction j, TJi is the over-potential of reaction i. The formation of the multi-electrode coupled system must accord with the two above-mentioned formulas. Mixed potential E can be gained by factual measurements, and have the following characteristics generally. (1) Total current of the reaction is zero. (2) In the multi-electrode reaction coupled system, the mixed potential E always locates between the highest and the lowest equilibrium potentia1. That is to say, at least there is an electrode equilibrium potential lower or higher than the mixed potential. (3) When the equilibrium potential is higher than the mixed potential E, the reactions occur towards cathode reaction. Mineral static potential or mineral electrode potential is an important parameter. It plays an important role in flotation. It has been demonstrated that the floatability of minerals has direct relation with electrode potential because the grindingflotation system is similar to the multi-electrode reaction coupled system. Therefore, it is important to study the variance rules of the mineral electrode potential under different conditions. ]

7.1.2

The Concept of Corrosive Current and Corrosive Speed

E,,a and E,,c represent the equilibrium potential of mineral anodic dissolution and cathode reduction of oxygen, respectively. E corr represents the mineral mixed potential in certain system. fo,a and fo,c are current density of anodic and cathode reaction, respectively. When the discharge is the controlled step of electrode reaction, according to electrochemistry theory, the equation can be described as following:

fa = fo,a exp

EK -Ee,a RT - exp

(7-4)

naaF

-exp

EK

Ee,c RT naaF -

(7-5)

where a is transmission factor, when potential is E corr 169

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

(7-6) I corr is corrosive current density, used to describe the corrosive speed of

minerals. Corrosive current value indicates the speed of electrochemistry reaction on the mineral surface.

7.1.3 The Corrosion Inhibitor, Inhibiting Corrosive Efficiency and Its Relationship with Collector Action Corrosion inhibitor is a chemical which prevents metal from corrosion in a medium at certain concentration. For sulphide minerals, due to the addition of collector and pH regulator, the hydrophobic film of a collector or hydrophilic film of hydroxyl forms on the surface of minerals in the process of flotation, which prevents sulphide mineral from being" corrosive further because of the poor film conductivity. The study of inhibiting corrosive efficiency may be helpful to understand the mechanism of sulphide minerals-collector-pH regulator system. Generally speaking, inhibiting corrosive efficiency can be expressed by the following formula:

(7-7) where ~ and ~, are polarization resistance in water system and in corrosion inhibitor system, respectively. Polarization resistance is resistance which inhibits electron transfer on the surface of sulphide mineral. The more the resistance is, the more difficult the reaction is. The higher the inhibiting corrosive efficiency is, the stronger the interaction between sulphide mineral and collector is.

7.2 Self-Corrosion of Sulphide Minerals Figure 7.1 is the polarization curves for pyrite, galena and sphalerite electrode in 0.1 mol/L KN0 3 at natural pH. Electrochemistry parameters obtained by the computer PARcal are listed in Table 7.1. The results indicate that the corrosive potential and the corrosive current of pyrite are the highest. The corrosive potential of galena is the lowest, while its corrosive current is lower than that of pyrite and higher than that of sphalerite. The corrosive potential of sphalerite is in between that of pyrite and galena and its corrosive current is the lowest. These results suggest that the oxidation to be easier in the order of pyrite> sphalerite> galena at given potentiaL The oxidation of pyrite would be the fastest. 170

Self-Corrosion of Sulphide Minerals

7.2 0.5

w: :r:: rJJ

-;;:

0.4

--ZnS _ . - FeS2

0.3

......... PbS

/ ~

---------- .

.~

0.2 0.1

~ 0.0

...... ,,::::::.~

-0.1

....

\ .......

-

.....

-0.2 -0.3

"" ,\

: .:.

-9

-8

-7

-6

-5

-4

-3

19l

Figure 7.1 Polarization curves of sulphide minerals electrode at natural pH (KN0 3 : 0.1 mol/L; unit of I: A/cm2) Table 7.1

Tafel parameters of sulphide minerals electrode at natural pH

Ieorr(~cm2)

Eeorr (mV, SHE)

b, (mV)

be (mV)

~(kQ)

Pyrite

10.78

187

124

125

6.2

Galena

3.45

-48

122

117

13.3

Sphalerite

0.13

42

129

120

48.6

Minerals

Figure 7.2 shows the electrochemistry impedance spectrum (EIS) of sulphide minerals electrode at natural pH under the corrosive potential. The ESI are in the x-y quadrant, x represents the resistance and y represents capacitance. It can be seen from Fig. 7.2 that three sulphide minerals appear as a single capacitive loop, suggesting that the electron transfer circuit on a mineral surface is equivalent to a parallel circuit of a capacitance and resistance as shown in Fig. 7.3. In Fig. 7.3, R; represents the solution resistance by comparing the electrode to a working electrode. C represents the double layer interface capacitance of sulphide minerals. RF represents the Faraday transfer resistance, which is generally equal to polarization resistance of a mineral. The polarization resistance reflected the difficult degree of electron transfer on a mineral surface. The polarization resistance can be determined according to the intersection point between capacitive loop and x-axis so that the longer the radius of capacitive reactance loops, the bigger of resistance of a mineral surface. It can be seen from Fig. 7.2 that the radius of capacitive reactance loop decreases and hence surface resistance descends in the order of pyrite, sphalerite and galena, showing that the oxidation products are formed on the mineral surface with more on pyrite. i.e. the oxidation of the three minerals descends in the order of pyrite, sphalerite and galena. 171

Chapter 7 Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals 40000,-------------, 2500 ZnS

30000

1

2000

,,-...,

~ 20000

o o

~

o

o

0 0

0

So a

0

o

cPo

10000

~ 1000

o

0 00

o

B

o

•

•

• •

••

•

•

500 •

;

o

OJ

1...0

1500

o

10000 20000 30000 40000 50000 Zr(n·cm 2) (a)

• 3000

1000

5000

Zr(n·cm2) (b)

...7000

7000,-------------, 6000 ,,-...,

N

S u

a

~

N~

5000 4000 3000 2000

•

•

• •

• •

•

•

PbS

•

1000

o

5000

10000

15000

Zr(n·cm2) (c)

Figure 7.2 EIS of sulphide minerals electrode at natural pH (KN03 : 0.1 mol/L)

C

Figure 7.3

The equivalent circuit for measure impedance of mineral

7.3 Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under Different Conditions 7.3.1

The Oxidation of Pyrite in NaOH Medium

Figures 7.4 and 7.5 are the polarization curves and EIS of pyrite under different concentration of the NaOH media. The results show that its corrosive potential move towards negatively and the corrosive current density decreases with the increase of NaOH concentration. The capacitance loop radius increases with the 172

7.3 Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

increase of NaOH concentration. The reaction resistance increases from 6000 in the absence of NaOH to 8500 in the presence of 10- 2 mol/L NaOH. EIS exhibits passivation characteristic. The results indicate the formation of surface oxidation products which prevents from transferring of electron, giving rise to the increase of surface resistance and the descent of corrosive current. With the increase ofNaOH concentration, the surface oxidation is stronger and the corrosive current density decreases more. 0.5 0.4 0.3

G3'

J:

V1

0.2 0.1

>=' 0.0

~ -0.1

-0.2

.... ~.=.~:~2-.~.... ,-, -Omol/L - - - 10-3 mol/L - - 5XI0-3 mol/L ....... 10-2 mol/L

,..

....

:-.. "-

~

.....

-0.3 -0.4 '---L--L--L--L--~~L--~'-----''-----'-----'"-----J -9 -8 -7 -6 -5 -4 -3 IgI

Figure 7.4 Polarization curves of pyrite electrode at natural pH and in NaOH medium (KN03 : 10- 1 mol/L; unit of I: A/cm 2) 12000 r - - - - - - - - - - - - - - - - - ,

9000 .--

1=u

g

• 0 mol/L o 10-3 mol/L o 5xlO-3 mol/L • 10-2 mol/L

6000

N3000

rtfo·o·0 • •

0

o

•

o

3000

0 .

o.

0 0 • .00 •

6000 9000 Zr(n·cm2)

12000

Figure 7.5 EIS of pyrite electrode under different NaOH concentration (KN0 3 : 0.1 mol/L)

Figure 7.6 is the EIS of pyrite under different potential conditions in NaOH solution. The relationship between polarization resistance and potential can be further demonstrated by Fig. 7.7. It can be seen from Fig. 7.6 and Fig. 7.7 that when the anodic polarization potential is between 50 and 330 m~ all the curves appear as a single capacitive reactance loop. But when the potential is between 50 and 250 mV, the capacitive reactance loop radius increased with the 173

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

enhancement of the electrode potential. This may be due to the formation of hydroxyl iron precipitate on the mineral surface. The surface resistance increases from 8700 to 11600 with the increase of potential from 50 to 250 mV and the growth of surface oxide film is the controlled step. When the potential is between 270 and 330 mY, the capacitive reactance loop radius decreases from 1100 to 6000 at potential from 270 to 330 mY, indicating that the dissolution of hydroxyl precipitate is related on dissolution speed. At potential > 270 m~ the polarization resistance decreases with the decrease of electrode potential, indicating that the dissolution of passivation film may be the controlled step of the electrode process. 14000..---------------,

10000.-----------------,

250mV 170mV • 120mV • 50mV

12000

o

1

10000

a

8000

;r 6000

o

o ; r 4000

00 0 0 0 0 0 0 0 0 •••• 0 00 0 0 •• .. • 0 0 • • • • • • 00 ••• 0

'a •

•

'a •••

..0. •

on

6000

4000

o

aDo

o

4000 2000

So

~

o

o

o 270mV o 300mV • 330mV

8000

o

•

•

•

•

.-

o

0 00 0 00 0 0, •

cP 2000

0

0

0 0 00.

0

0

0 0

•

8000 • - '"'12000

o

o

•

•

QO. •

o

o

L...C.ffE-e-~--i---'--0 ""b4000

o --...L()---of--l

8000

Zr(Q'cm2)

Zr(Q'cm 2)

(a)

(b)

Figure 7.6 EIS of pyrite electrode under different potential condition in NaOH solution (NaOH: 0.01 mol/L)

15000 14000 13000 12000

§

11000

r:
9000 8000 7000 6000

o

.............--"----'---"--""O""-""""--...r....-....l~__I__"____'____'__""""'___~___'

50

100 150 200

250 300

350 400

E(mV, SHE)

Figure 7.7 Relationship between polarization resistance and potential of pyrite electrode in NaOH solution (NaOH: 0.01 mol/L)

174

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

The oxidation of Fe8 2 presents passivation characteristic in sodium hydroxide solutiondue to the following reactions: (7-8)

Fe8 2 +80H- ~Fe(OH)2 +280~- +6H+ +10e-

(7-9) (7-10)

Fe8 2 +IOOH- ~Fe(OH)2 +280;- +8H+ + 14e-

(7-11)

The formation of hydroxyl precipitate prevents from the transfer of electron especially between oxygen and the mineral surface. As a result of all these processes, EIS represents passivation characteristic. And the corrosive potential moves towards negatively, the surface resistance increases, and the corrosive current decreases. The formation of surface hydroxyl iron precipitates makes the pyrite surface very hydrophilic.

7.3.2 Oxidation of Pyrite in Lime Medium Figures 7.8 and 7.9 are the polarization curves and EI8 for pyrite at natural pH and in the lime medium, respectively. Obviously, after adding lime, the corrosive potential of pyrite electrode moves towards negatively about 150 mV and the corrosive current density decreases from 10.7 JlA/cm2 to 6.2 ~cm2. The anodic and cathodic slope has almost no change. Whereas, the capacitive reactance 0.5r-------------------, 0.4 0.3 ~

0.2

~

0.1

~

';>

~ 0.0

Pyrite

- ----------=---:..:Lime+pyrite

-0.1

-,

,

\ \

\

-0.2

\

\

-0.3 ~....L.--9

_

_ _ L _ _ - - - L - _ - - - - - L_ _

-8

-7

-6

......

-

...L...___~_

-5

-4

___J

-3

IgI

Figure 7.8 Polarization curves of pyrite electrode at natural pH and at pH = 12 modified by lime (KN03 : 0.1 mol/L; unit of I: Azcm')

175

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals 12000

Lime+pyrite • Pyrite

10000

o

8000 0

6000

0

0

4000 o

2000

o

o

o

o

0

o

o ••

•

•••

0

o •• o ••

,.

0---

o

•

4000

•

12000

8000

Zr(Q crn-) 0

Figure 7.9 EIS of pyrite electrode at natural pH and at pH = 12 modified by lime (KN0 3 : 10- 1 mol/L)

loop radius increases evidently in the presence of lime. The electrode resistance increases about 5600 Q, i.e. from 6200 Q to 11800 Q. The EIS exhibits passivation characteristic indicated the formation of surface oxidation products. And the electrode surface resistance about 11800 n in the lime medium is much higher than that of about 8500 n in NaOH solution comparing with Fig. 7.5. It demonstrates that pyrite surface is strongly oxidized in the lime medium. Further, the EIS of pyrite under different polarization potential in the lime medium are measured and shown in Fig. 7.10. The relationship between polarization resistance and potential can be demonstrated in Fig. 7.11. It can be seen from Fig. 7.10 and Fig. 7.11 that when the anodic polarization potential is between 20 and 350 mY, all the curves appear as a single capacitive reactance loop. But surface process of pyrite electrode is controlled by a quite different

,--,

14000....---------------,

14000,-----------------,

12000

12000

o 250mV

° 170mV

10000

1

·90mV -20mV

8000

a

o

o

~ 6000

2000

1

o

a

o o

0 0

o 0 o. ••

D

0 0

0

0

0

•• ". ••

1.

]. o '0"

4000

8000 Zr(Q crri2) (a) o

280mV

- 350mV

8000 0

o

0 0

°

0

:..•••

•

DO 00 0 0

4000

°

0

2000

0

12000::

0

o

0

0

0

• : •

'6 ~

10000

~ 6000

0

Do

4000

,--,

o

° 330mV

00

0 0 0 •

o. "

ol(, •

1· "0 -

4000

•

0

0

°

•

o o

o

•

8000 .. Zr(Q cm-) (b) 0

Figure 7.10 EIS of pyrite at different potential in the lime medium (pH = 12; KN0 3: 0.1 mol/L)

176

0

o

12000

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ... 15000 14000 13000 12000

§ Q..

~

11000 10000 9000 8000 7000 6000

o

L....-..L..----L...---L..-....JI..-.J---L-""---'--.J.---L...-.L..-.L.~-l-L..--L...-L.-...J--.J

50 100 150 200 250 300 350 400 450 E(mV, SHE)

Figure 7.11 Relationship between polarization resistance and potential of pyrite in the lime medium

process. When the potential is between 20 and 250 mV, the capacitive reactance loop radius increases with the enhancement of the electrode potential. This indicates the formation of hydroxyl iron precipitate on the mineral surface. The surface resistance is increased from 11500 Q to 14600 Q with the increase of potential and the growth of surface oxide film is the controlled step. When the potential is between 280 and 350 mY, the capacitive reactance loop radius decreased with the enhancement of electrode potential indicates that the dissolution of hydroxyl precipitate occurs and capacitive reactance loop radius is dependent on the dissolution speed. Polarization resistance decreases from 14400 Q to 9500 Q with the decrease of the electrode potential from 280 mV to 350 mV. Here, the dissolution of the passivation film is the controlled step of the electrode process. Comparing EIS for pyrite in lime medium and different polarization potential (Figs. 7.10 and 7.11) with those in sodium hydroxide solution (Fig. 7.6 and Fig. 7.7), it can be seen that at a potential of about 50 mV the surface resistance in the NaOH solution (about 8700 Q) is lower than that in the lime medium (about 12000 Q). It means that besides the formation of hydroxyl precipitate on the surface of pyrite in lime media similar to that in NaOH media, there may be other surface products being formed, which may be due to the following reactions: Ca2+ + S024

=

CaOH+ + SO~- =

CaSO 4 CaSO 4 +OH-

(7-12) (7-13)

The formation of hydroxyl iron and calcium sulphate precipitates makes pyrite surface very hydrophilic and inhibits other electrochemistry reaction on pyrite like collector giving rise to the depression of pyrite. The depression effect of lime on pyrite may be stronger than that ofNaOH. 177

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

7.3.3 Corrosive Electrochemistry Study on Interactions between Collector and Pyrite 1. PyritelXanthate Interaction Figure 7.12 is the polarization curves of pyrite electrode in xanthate solution with different concentration for dipping for 48 hours. Electrochemistry parameters determined by the computer PARcal are listed in Table 7.2. Inhibiting efficiency can be calculated by Eq. (7-7), Rp' is the polarization resistance after adding collector, Rp is the polarization resistance without collector. It can be seen from Fig. 7.12 and Table 7.2 that, with the increase of xanthate concentration, corrosive potential and corrosive current of the pyrite electrode decrease gradually while polarization resistance increases, indicating the formation of surface oxidation products. 0.5 . . . . - - - - - - - - - - - - - - - - - - ,

0.4 0.3

0.2

G:i' 0.1 ~

~ 0.0

r.tr -0.1

Omol/L - - - 1XI0-4 mol/L - - 3xl0-4 mol/L ....... 5xl0-4 mol/L

-0.2 -0.3 -0.4

'---",,--",",,---'----'---'--.......&...-----&...--.1....----'-----'------,"-----''-----'

-9

-8

-7

-6

-5

-4

-3

19l

Figure 7.12 Polarization curves of pyrite electrode at different xanthate concentration (KN0 3 : 0.1 mol/L; pH = 7; unit of I: A/cm 2) Table 7.2 Corrosive electrochemistry parameters of pyrite at different xanthate concentration Collector

Xanthate

c (mol/L)

17 (%)

Rp(kn)

Ecorr(mV, SHE)

I corr (JlA/em')

0

-

6.2

187

10.78

10- 4

46.09

11.5

175

5.96

2 x 10- 4

53.73

13.4

153

5.2

5 x 10- 4

57.53

15.6

122

5.99

Icorr - corrosive current; Ecorr - corrosive potential; Rp- polarization resistance; 7]- inhibitefficiency.

Figure 7.12 is the EIS of pyrite under different xanthate concentration. Figure 7.12 exhibits two capacitive reactance loops existing in the pyrite-xanthate system 178

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

different from the self-oxidation of pyrite. The capacitive reactance loop with high resistance value represents the process of charging or discharging between electrode surface and double layer in solution. Low value capacitive reactance loop is caused by specific adsorption of species on the electrode surface. 15000.----------------------,

• 0 mol/L

o 10-4 mol/L

12000

l' o

° 2XI0-4 mol/L • 5XI0-4 mol/L

9000

a

~ 6000 3000

o

•

/~: •

3000

• •0 •0 0 0

•

0

0

0

0

0

•

• 0

0

• 0

•-

6000 9000 Zr(U'cm2)

0

0

o

• 0 0

•

o•

o-a·

u12000 '"15000

Figure 7.13 EIS of pyrite electrode at different xanthate concentration (KN0 3: 10- 1 mol/L, pH = 7)

In xanthate solution, FeS2have reactions as follows: (7-14) (7-15) (7-16) It may be derived that high value capacitive reactance loops are induced by electron transfer from Eq. (7-14), i.e. the chemisorption of xanthate ion on the pyrite surface, and low value capacitive reactance loops are induced by the adsorption and desorption of X2 from Eqs. (7-15) and (7-16). The capacitive reactance loop radius increases with the increase of xanthate concentration, indicating the thickening of the collector film on pyrite surface, increasing conduction resistance and weakening dissolution of pyrite. It suggests that the adsorption of xanthate on the pyrite surface may undergo several steps such as the adsorption of xanthate ion, the formation of dixanthogen due to the oxidation of the adsorbed ion and the increase of dixanthogen film with the increase of xanthate concentration. Figure 7.14 is the EIS of pyrite at different polarization potential with xanthate as a collector. The relationship between polarization resistance and potential can be further demonstrated by Fig. 7.15. 179

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

14000 12000 10000

• • •

~

8000 e o

("I

a~

••

6000 4000 2000

o

I

• •cP .~p

• C'

12000

• 320mV • 260mV o 180mV o 120mV ~

o

8000

e

("I

•

•

• 340mV 400mV

10000

~ 6000

• •

oo

0

~

•

4000

tJ

0

~

0

4000

8000

• • o

r

•

•

0 0

«)

2000

•

0

0

0

DO

12000

•••

• 000 ·0 .0

0 0

••

~

0

Zr(U'cm2) (a)

t"'I

4000

8000

f2000

Zr(U'cm2) (b)

Figure 7.14 EIS of pyrite at different potential with xanthate as the collector (pH = 7; xanthate: 10- 4 mol/L) 13000 12000 11000

§ 0-

cc:::

10000 9000 8000 7000 6000 100 150 200 250 300 350 400 450 E(mY, SHE)

Figure 7.15 Relationship between polarization resistance and potential of pyrite (pH = 7; xanthate collector: 10- 4 mol/L)

Figure 7.14 illustrates that in the initial stage of polarization of the pyrite electrode in xanthate solution at about 120 mY, the radius of high value capacitive reactance loop increases with the increase of the polarization potential and reaches the maximum at 320 mV, indicating that the oxidation of xanthate increases gradually and collector film on pyrite surface becomes thicker. It increases the conduction resistance and the growth of collector film is the controlled step resulting in pyrite surface hydrophobic. When the polarization potential increases from 320 mV to 400 mY, the capacitive reactance loop radius decreases, indicating the decrease of transferring conduction resistance as can be seen in Fig. 7.15. It belongs to the step of film dissolution. Capacitive reactance loop radius decreases obviously when the potential is larger than 400 mY, at where the collector film falls off and the anodic dissolution of pyrite occurs. The controlled step is the anodic dissolution of pyrite and the surface becomes 180

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

hydrophilic. The results in Figs. 7.14 and 7.15 account for the flotation potential range of pyrite and voltammetric results with xanthate as a collector in Figs. 4.23 and 4.24 where the formation of dixanthogen and the growth of the collector film takes place. 2. PyritelDithiocarbamate Interaction Figure 7.16 is the polarization curves of the pyrite electrode in dithiocarbamate solution at different concentration for dipping for 48 hours. Electrochemistry parameters determined by the computer PARcal are listed in Table 7.3. It can be seen from Fig. 7.16 and Table 7.3 that the corrosive potential of pyrite electrode decreases gradually from 187 to 160 mV and the corrosive current decreases from 10.78 to 6.01 flA/cm2 without or with the DDTC addition of 5 x 10-4mol/L, while polarization resistance increases from 6.2 to 10.1 ill with the increase of dithiocarbamate concentration. It indicates the formation of surface oxidation products. Comparing with xanthate, DDTC has less effect on corrosive potential, current and polarization resistance. It indicates that collector function of DDTC on pyrite is less than that of xanthate. 0.5

-Omol/L - - - lx10-4 mol/L - - 3xI0-4 mol/L ....... 5x 10-4 mol/L

0.4

GJ 0.3

:I:

sr:

>

~

0.2

- --=:.::

.......::-

-=-=.~-=---:~.r~

0.1

"

'. \ ".

0.0 -0.1

-9

-8

-7

-6

\

\ ..... \,

".

-5

"','--4

-3

19l

Figure 7.16 Polarization curves of pyrite electrode at different dithiocarbamate concentration (KN03 : 0.1 mollL; pH = 7; unit of I: A/cm2) Table 7.3 Corrosive electrochemistry parameters of pyrite at different collector concentration Collector

DDTC

c (mollL)

1] (%)

Rp(kQ)

Ecorr(mV, SHE)

Icorr(~cm2)

0

-

6.2

187

10.78

35.74

9.5

179

6.34

10- 4 4

36.08

9.7

164

6.13

5 x 10- 4

38.61

10.1

160

6.01

2 x 10-

181

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

Figure 7.17 is the EIS of pyrite electrode under different DDTC concentration. If only according to the measured rest potential (reversible potential for oxidation to thiouram disulphide (Finkelstein and Goold, 1972)), the products of the interaction of pyrite with sodium dimethyl dithio carbamate are predicted to be thiouram disulphide. However, the EIS of the pyrite electrode in the Fig. 7.17 shows a single capacitive reactance loop characteristic, indicating that the interaction between DDTC and pyrite may be dominated by the adsorption of DDTC on the pyrite surface similar to Eq. (7-14) and the oxidation ofDDTC into disulphide (DDTC)2 is relatively difficult comparing with xanthate, indicating weak collecting ability of dithiocarbamate on pyrite. 12000.-----------------, • 0 mol/L o 10-4 rnol/L o 2x10-4 mol/L 9000 • 5 xt 0-4 mol/L

l'o a 6000 ~

...

.i 3000 r~ o

~

•• •••

~oo 0 80 0

2000

Cb

0

0

0

0

4000

•

•

~8 0

6000

•

d'

d° 0

8000

~

•

10000

Zr(Q·cm2)

Figure 7.17 EIS of pyrite electrode in different DDTC concentration (KN03 : 0.1 mollL; pH= 7) 10000,..---------------, 8000

l'o

6000

a

•

~ 4000 2000

8000

• 350mV • 260mV o 150mV

• •• • • •• ••

.~oo

I

0

0

0

0

4tJ

o ..

N

2000

•

~

• •

••

• • •

•

'OJ

0

-aD

•

0

fJ

0

•

• • r# reo 0 0

2000

__

4000 6000 8000 ~ i 0000 Zr(O·cm2) (a)

8

ao 4000 o

o rJI

6000 ~

o ••

tJ

• 390mV o 450mV

n

4000

8000

:

Zr(O·cm2) (b)

Figure 7.18 EIS of pyrite electrode at different potential with DDTC as the collector (pH = 7; DDTC: 10- 4 mol/L)

Figure 7.18 is the EIS of the pyrite electrode at different polarization potential with DDTC as a collector. The relationship between polarization resistance and 182

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

potential can be further demonstrated by Fig. 7.19. It can be observed from Fig. 7.18 and Fig. 7.19 that in DDTC solution, the capacitive reactance loop radius increases with the increase of polarization potential and reaches the maximum at 350 mY, indicating that the adsorption ofDDTC increases gradually and the adsorbed collector film on pyrite surface becomes thicker. When the polarization potential increases to 390 mV, capacitive reactance loop radius and the transferring of conduction resistance decrease, indicating the desorption of adsorbed collector. Capacitive reactance loop radius decreases obviously when the potential is larger than 450 mY, suggesting that the collector film falls off and the anodic dissolution of pyrite occurs. This potential is higher than that in the xanthate system. 13000 12000 11000

§ Q.

~

10000 9000 8000 7000 6000 100 150 200 250 300 350 400 450 E(mV, SHE)

Figure 7.19 Relationship between polarization resistance and potential of pyrite (pH = 7; DDTC collector: 10- 4 mol/L)

7.3.4 Interaction between Collector and Pyrite in High Alkaline Media Figures 7.20 and 7.21 are the polarization curves of pyrite interaction with collector at pH = 12 modified by NaOH and lime, respectively. It can be seen from Fig. 7.20 that the corrosive electrochemistry parameters do not change too much whether collector is added or not at pH = 12 modified by NaOH. In the presence of xanthate, corrosive current decreases slightly. Xanthate shows certain inhibiting corrosive function, i.e. adsorption on pyrite even at high pH media. Hence, xanthate still shows some collecting ability on FeS2 at pH= 12 modified by NaOH. Figure 7.21 shows that the DDTC has no effect on the corrosive current and corrosive potential at pH = 12 modified by lime. The inhibiting corrosion action of DDTC is much lower than that of xanthate, i.e. no adsorption of DDTC on pyrite at high pH modified by lime. It demonstrates that DDTC has no collecting action on pyrite at pH = 12 modified by lime, accounting for that

183

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

DDTC has better selective action than xanthate in flotation separation ofPb-Zn-Fe sulphide ores in high lime medium. Figures 7.20 and 7.21 also show that in the presence of collectors, the decreasing degree of the corrosive current of pyrite electrode in the NaOH system is slightly bigger than that in the lime system. It demonstrates that the inhibiting corrosive function and the collecting ability of collector on pyrite in the NaOH system is stronger than that in the lime system. That is to say, xanthate has certain collecting function on pyrite at pH = 12 modified by NaOH, while the collecting function decreases greatly in the lime system. Lime may be the effective depressant for pyrite with xanthate and DDTC as collectors. 0.5 ~--------------. NaOH+DDTC - - NaOH+BX ·······NaOH

0.4

0.3

rr-:

UJ

:c u:

';> 0.2 ~

.................................. ~

-

.............

0.1

0.0 -0.1

e ••

,r.. ,,

<, •• ~

.•..

.

'----'--_--'---_--'--_~_~_-..I--_--'

-9

-8

-7

-5

-6

-4

-3

IgI

Figure 7.20 Polarization curves of pyrite electrode interaction with collector at pH = 12 modified by NaOH (KN0 3: 0.1 molJL; collector concentration: 10-4 molJL; unit of I: AIem')

0.5~-----------------,

0.4 0.3

0.2

GJ

Lime+BX ...... Lime+DDTC - - Lime

I 'I

~ 0.1 ';>

~

~ O.O~-_..........~........aa:~ .. ~

-0.1 -0.2

..~

-0.3 L..---~_......I...-_~_~_~_--'--_--' -9 -8 -7 -6 -5 -4 -3 IgI

Figure 7.21 Polarization curves of pyrite electrode interaction with collector at pH = 12 modified by lime (KN03: 0.1 mol/L; collector concentration: 10- 4 mol/L; unit of I: AIcnr')

184

7.3

Corrosive Electrochemistry on Surface Redox Reaction of Pyrite under ...

Figures 7.22 and 7.23 are the EIS of pyrite interaction with collector at pH= 12 modified by NaOH and lime respectively. Figure 7.22 shows that the radius of the capacitive reactance loop is, respectively, 8700 Q without collector, 9400 Q by the addition of xanthate and 10000 n by the addition of dithiocarbamate. The small increase of the radius of the capacitive reactance loop at pH = 12 modified by NaOH when adding xanthate and dithiocarbamate, shows that the two collectors still have certain inhibiting corrosion action and hence adsorption on pyrite. The action of xanthate is some stronger than that of DDTC because the radius of the capacitive reactance loop in the presence of xanthate is slightly larger than that in the presence ofDDTC. However, Fig. 7.23 demonstrates that at pH= 12 modified 12000....----------------., • NaOH+BX • NaOH+DDTC o NaOH

9000

e

~

a 6000 u

•

~

•

3000

• ., • -

o •

oe

o

•o •• o •

3000

6000

•

9000

12000

Zr(Q'cm2)

Figure 7.22 EIS of pyrite interaction with collector at pH = 12 modified by NaOH (KN0 3 : 0.1 mol/L; collector concentration: 10- 4 mol/L) 14000 12000

o Lime+BX

10000

Lime+DDTC • Lime o

~

N

8o 8000

a

~

6000 4000 2000

o

4000

8000

12000'"

Zr(Q'cm2)

Figure 7.23 EIS of pyrite interaction with collector at pH = 12 modified by lime (KN0 3 : 0.1 mol/L; collector concentration: 10- 4 mol/L) 185

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

by lime, the radius of capacitive reactance loops changes little in the presence of xanthate and dithiocarbamate, indicating that at pH = 12 modified by lime, the oxidation products on pyrite surface may be almost the same whether in the absence or presence of collectors. It demonstrates that xanthate and dithiocarbamate exhibit very weak inhibiting corrosion action and hence collecting ability on pyrite at high pH modified by lime.

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under Different Conditions

7.4.1

The Oxidation of Galena in NaOH Solution

Figure 7.24 is the Tafel curves of the galena electrode at natural pH and at pH= 12 modified by NaOH. It can be seen from Fig. 7.24 that the corrosive potential of galena electrode decreases a little and the corrosive current decreases from 3.45 to 3.05 JlA comparing the results at pH= 12 modified by NaOH with natural pH. The inhibiting corrosive efficiency is about 17.9%. The EIS of galena electrode is given in Fig. 7.25. It shows single capacitive reactance loop and the radius of the loop changes little in the presence or absence of NaOH, indicating that NaOH has little inhibiting function on the dissolution of galena. These results suggest that, at natural pH and high pH modified by NaOH, the oxidation products on the galena surface have almost the same influence on the surface conductivity. 0.3r-----------------,

0.2 0.1 G:;' ~

- - PbS+NaOH -PbS

0.0

':>

~ -0.1

-0.2 -0.3 -8.0 -7.5 -7.0

-s.s -e.o -5.5

-5.0 --4.5 --4.0 -3.5

IgI

Figure 7.24

Polarization curves of galena electrode at natural pH and pH = 12 modified by NaOH (KN0 3 : 0.1 mol/L; unit of I: Azcnr')

186

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under ... 10000.....-----------------, • PbS

9000

o PbS+NaOH

8000

Eo

~

a

~

7000 6000 o

5000

0

4000

o

•

•

0

•

3000

o

•o

2000 1000

o o

o

5000

10000

]5000

ZrCQ'cm2 )

Figure 7.25

EIS of PbS at natural pH in NaOH medium (KN03 : 0.1 mol/L)

7.4.2 The Effect of Lime on the Oxidation of Galena The polarization curves at different pH and the EIS of galena electrode are, respectively, given in Fig. 7.26 and Fig. 7.27. It follows from Fig. 7.26 that the corrosive potential reduces about 30 mV and corrosive current decrease from 3.45 to 2.14 J.1A when pH increases from natural pH to pH = 11.8 by adding lime. The inhibiting corrosive efficiency of lime is about 42.05% higher than that of Na(OH) about 17.9%, suggesting the formation of surface products and inhibiting corrosive action of lime on galena, which is stronger than NaOH. The surface oxidation products are some different at natural pH in lime medium. According to the theory of corrosive electrochemistry, the negatively towards 0.3, - - - - - - - - - - - - - - - - - - , 0.2 0.1

w: 0.0 :c ir: -;;: ~·-0.1

- - PbS+lime -PbS

'I

----===-..

",.

<,

-0.2 -0.3 -8.0 -7.5 -7.0

-s.s -s.o -5.5

.,,-

,;

",

\

/

,

/

<,

/

'I

I

........

-5.0 -4.5 -4.0 -3.5

19I

Figure 7.26 Polarization curves of galena electrode at natural pH and pH = 11.8 modified by lime (KN0 3 : 0.1 mol/L; unit of I: Azcnr') 187

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

10000~-------------'

9000

• PbS

o PbS+lime

8000 ~

§

7000 6000

o

9"

5000 N- 4000

0

0

o·

3000

00

2000 1000

o

o •

0

•

o

•

o

•

0

o·

o

00

~

o

5000

10000

.

o

15000

Zr(Q·cm2)

Figure 7.27 EIS of galena electrode at natural pH and pH = 11.8 modified by lime (KN0 3: 0.1 mollL)

shift of corrosion potential by adding lime suggests that the cathode reaction is retarded by some non-electroactive materials. It makes O2 not contact effectively with the electrode surface, giving rise to the decrease in the reduction efficiency of O2, further inhibiting the corrosion rate of galena. In Fig. 7.27, there appears a single capacitive reactance loop in different pH media. The capacitive reactance loop radius is bigger in the lime medium than that without lime. The reaction resistance increases from 14000 in the absence of lime to 15000 in the presence of lime. EIS exhibits passivation characteristic. The results indicate the formation of surface oxidation products which prevent the transferring of electron, giving rise to the increase of surface resistance and descending of corrosive current. It may be mainly because of the following reactions on the surface of galena in the lime medium besides Eq. (7-12): PbS+20H- ~Pb(OH)2 +S+2e-

(7-17) (7-18)

Pb 2+ + S02= 4

PbSO 4

(7-19)

Figure 7.28 is the EIS of the galena electrode at different potential in the lime medium. The relationship between polarization resistance and potential is presented in Fig. 7.9. The EIS of the galena electrode can be divided into three stages according to the different characters of the surface oxidation film. When the potential is between - 70 and 300 mV, capacitive reactance loop radius and polarization resistance increases slowly due to the formation of surface oxidation products, and the growth of surface oxidation film is the controlled step of the 188

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under ...

electrode process. From 300 to 400 mv, polarization resistance decreases gradually and the dissolution of oxidation film is the controlled step of electrode process. Polarization resistance decreases sharply after 400 mV, the oxidation film falls off and the dissolution of galena is the controlled step of the electrode process, which may be corresponding to the formation of HPbO~. (7-20) (7-21)

10000~-------------'

,-...

Eo a

• -70mV o 100mV o 200mV ·300mV

9000 8000 7000

l'o 6000

6000 5000 ~ 4000 3000 2000 1000

o

10000,--------------, • 350mV o 400mV 8000 o 450mV

a~ 4000 2000

5000

10000

15000

0

2500 5000 7500 10000 12500 15000

Zr(Q·cm 2) (a)

Zr(Q·cm2) (b)

Figure 7.28 EIS of galena electrode at different potential in lime medium (KN0 3 : 0.1 mollL) 16000.------------------. 15000 14000 13000

~ 12000 ~

11000 10000 9000 8000 -100

L - - _ - i . . . - _ - - L_ _. . L . . . - _ - - l . - _ - - - - - L . . . _ - - - - l

0

100

200

300

400

500

E(mY, SHE)

Figure 7.29 Relationship between polarization resistance and potential of galena electrode in lime medium

189

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

7.4.3 Corrosive Electrochemistry Study on Interactions between Collector and Galena 1. GalenalXanthate Interaction Figure 7.30 is the polarization curves of the galena electrode in xanthate solution with different concentration. Figure 7.31 is the EIS of the galena electrode in xanthate solution with different concentration. The corrosive electrochemistry parameters are listed in Table 7.4. From Fig. 7.30 and Table 7.4, it can be seen that, after adding xanthate 5x 10- 4moI/L, the corrosive potential of galena electrode decreases from -48 to -94 mY, the corrosive current of the galena electrode decreases from 3.45 to 0.99 ~cm2, the polarization resistance increases to 18.7 ill and the inhibiting corrosive efficiency increases to 28.34. Figure 7.31 shows that the EIS of galena electrode appears to have single capacitive reactance loop characteristic and the radius of the capacitive reactance loop increases with the increase of collector concentration. 0.4 , . - - - - - - - - - - - - - - - - - - , ----- - ........

0.3 0.2

Omol/L 10-4mol/L 2x10-4 mol/L 5x10-4 mol/L

/

v/:

f.: ~:

0.1

GJ ~ 0.0 ;;;:

..

~~

A~"

- - - ---

~-0.1

.7:.:":":".:":":',:"Ff:

- ...-;.. ~ :~:~':~.~

~.'

~.'

'

..

' ~,

-0.2

..~~~ .. ..... ..

_-

~ ' ~.:-::

-0.3

-0.4 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 19I L-------L-_-L.------L_----L...-_--L..-----L_---I-_..L------l

Figure 7.30

Polarization curves of galena electrode at different xanthate concentration (KN0 3 : 0.1 mol/L; pH = 7; unit of I: A/cm2) Table 7.4 Corrosive electrochemistry parameters of galena electrode in xanthate solution with different concentration

Collectors

c (mol/L) 0

BX

190

1] (%)

-

Rp(kn)

Ecorr(mV, SHE)

I corr (~cm2)

13.4

-48

3.45

10-4

18.4

16.3

-68

1.46

2 x 10-4

27.47

18.2

-77

1.25

5 x 10-4

28.34

18.7

-94

0.99

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under ... 13000 12000 11000 10000 9000 ,,-.... N 8000 8 o 7000 a 6000 ~ 5000 4000 3000 2000 1000

• 0 mol/L o 10-4 mollL o 2xl0-4 mol/L • 5 Xl 0- 4 mol/L

• ••

• .0 0

}

0

• • 0

0 0

.0

0

• 5000

•

0

• 0

~

o

•

•

0

0

0

0

•

•

0

10000

0

•

15000

• 20000

Zr(Q'cm2)

Figure 7.31 EIS of galena electrode in xanthate solution with different concentration (KN0 3: 0.1 mol/L; pH = 7)

The above results demonstrate that the formation of surface products of the interaction between xanthate and galena increases with the increase of xanthate concentration and xanthate has stronger inhibiting corrosive action on galena.· The single capacitive reactance loop characteristic indicates that the lead xanthate may be the main oxidation product on galena surface, which is responsible for surface hydrophobicity. The amount of formation of lead xanthate will be enhanced with the increase of xanthate concentration based on the change of corrosive potential and current. Figure 7.32 is the EIS of the galena electrode at different polarization potential in xanthate solution. The relationship between polarization resistance and potential can be further demonstrated by Fig. 7.33. Performance of galena interaction with xanthate is different at different potential. From -50 to 300 mV potential ranges, the radius of capacitive reactance loop is enhanced with the polarization potential, 13000 12000 11000 10000 ,,-.... 9000 N 8o 8000 7000 C: 6000 ~ 5000 4000 3000 2000 co II 1000

• -50mV

,.

o

•

0

0 0

0 0

•

0

0

•

0 •

10000

•

~ 4000

D

Zr(Q'cm2) (a)

8o

a

• 0

5000

o 400mV o 450mV

(;" 6000

• 300mV

•• • • • ~'bD~

• 350mV

8000

o 100mV o 200mV

0

•

DO

15000

.~

• • 20000

2000

•

0

<>0

••

0 0

o

0

0 0

• D

0 0

•

0

•

.0

CP

0

00

0

0

2500

5000

0

•

7500 10000 12500 15000 Zr(Q'cm2) (b)

Figure 7.32 EIS of galena electrode at different potential with xanthate as the collector (KN03 : 0.1 mol/L; xanthate: 10-4 mol/L)

191

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals 20000

~----------------,

18000 16000

~ ~

14000 12000 10000 8000 -10

L...--1..L....--......&.....---J..----L..----L----'------L----...lIL.---'----......&.....---&----l

0

100

200

300

400

500

E(mV, SHE)

Figure 7.33 Relationship between polarization resistance and potential of galena electrode

the growth of the collector film PbX 2 is the controlled step of the electrode process. Within this potential range, xanthate may exhibit stable action and good collecting property on galena. Above this potential range (> 350 mY), the polarization resistance and capacitive reactance loop radius decrease evidently with the increase of potential, which may be attributed to the film dissolution of PbX 2• The controlled step of electrode process is determined by the dissolution of the collector film and the further oxidation of galena, which may result in the falling off of the collector film and galena loses its floatability. The EIS of the galena electrode may explain the results in Fig. 4.6.

2. GalenalDithiocarbamate Interaction The polarization curves and the EIS of the galena electrode in dithiocarbamate solution with different concentration are, respectively, presented in Fig. 7.34 and Fig. 7.35. The corrosive electrochemistry parameters are listed in Table 7.5. It can be seen from Fig. 7.34 and Table 7.5 that, in the absence of dithiocarbamate, the corrosive potential, corrosive current and polarization resistance of the galena 2 electrode are, respectively, -48 mY, 3.45 JlA/cm and 13.4 ill. By the addition of 4 5 x 10- mol/L dithiocarbamate, the corrosive potential of the galena electrode 2 decreases to -99 mY, the corrosive current decreases to 1.21 JlA/cm and the polarization resistance increases to 18.3 ill. The inhibiting corrosive efficiency is 27.32. It indicates the formation of surface products through the interaction between galena and collector. Figure 7.35 shows that the EIS of the galena electrode appears to have single capacitive reactance loop characteristic and the radius of the capacitive reactance loop increases with the increase of collector concentration. It demonstrates that the formation of surface products of the interaction between dithiocarbamate and galena increases with the increase of DDTC concentration. The single capacitive reactance loop characteristic indicates that the lead dithiocarbamate salt may be the dominant oxidation product on the 192

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under ... 0.3

--Omol/L - - - - 10-4 mol/L - - - 2x10-4 mol/L ........ 5x10-4 mol/L

0.2 0.1

w:

0.0

~ -0.1

";>

~ -0.2

-0.3

-0.4 -0. 5 '-----'--_~____L

_

_ _ _ ' _ _ _ . . L . . . _ - - - ' - _ . . . . l . . . . . - _ _ _ _ L_

___'__._I

-9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 19l

Figure 7.34 Polarization curves of galena electrode in DDTC solution with different concentration (KN0 3 : 0.1 mol/L; pH = 7; unit of I: A/cm 2) 13000 12000 11000 10000 ,,-..., 9000 N 8o 8000 7000 a 6000 ~ 5000 4000 3000 2000 1000

o

• 0 mol/L o 10-4 mol/L • 2XI0-4 mol/L o 5xt 0-4 mol/L

0

~o'f!:

0.

0

0

• • •

• 0

0

0

•

J 5000

•

0

•

0

•

0

10000

•

0

0 0

•

0

15000

20000

Zr(Q'cm2)

Figure 7.35 EIS of PbS at different DDTC concentration (KN0 3 : 0.1 mol/L; pH = 7) Table 7.5 Corrosive electrochemistry parameters of galena electrode in DDTC solution with different concentration Collector

c (mol/L)

0

-

10- 4 DDTC

17 (%)

Rp (Jill)

Ecorr(m~ SHE)

Icorr (JlAIcnr')

13.4

-48

3.45

13.15

15.2

-71

1.67

4

18.79

16.5

-81

1.42

5 x 10- 4

27.32

18.3

-99

1.21

2 x 10-

193

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

galena surface, which is responsible for surface hydrophobicity. The amount of formation of collector salt will be enhanced with the increase of DDTC concentration based on the change of corrosive potential and current with DDTC concentration. Comparing Table 7.4 and Table 7.5, it is obvious that the corrosive potential and the current of the galena electrode are lower in xanthate solution than in dithiocarbamate solution. The polarization resistance and the inhibiting corrosive efficiency in xanthate solution are higher than that in DDTC solution. It indicates that xanthate has stronger inhibiting corrosive action on galena than dithiocarbamate. Figure 7.36 is the EIS of the galena electrode at different polarization potential in DDTC solution. The relationship between polarization resistance and potential can be shown in Fig. 7.37. It is obvious that the interaction between galena and dithiocarbamate also shows different behavior at different potential. When the polarization potential ranges from -50 to 350 mY, the growth of collector film PbD2 will be the controlled step of electrode process because the radius of capacitive reactance loop, i.e. the polarization resistance is enhanced with the polarization potential. Within this potential range, the action of dithiocarbamate on galena can take place to form collector salt. Above this potential range, the polarization resistance and capacitive reactance loop radius decreased and the collector film dissolution occurs. The controlled step of the electrode process may be determined by the film dissolution and the galena oxidation. The collector film may detach from the surface and the galena surface may be oxidized to form hydroxyl compound. Comparing Fig.7.37 with Fig.7.33, the potential range of the interactions between collector and galena is wider for dithiocarbamate than xanthate. 13000 12000 11000 10000 9000 ~

N

e o

a

~

~QOO

7000 6000 5000 4000 3000 2000 1000

o

,e

• -50mV o 100mV o 200mV • 350mV

o 380mV • 420mV o 450mV

8000 6000

e o

N

~'b

••0 •0 •0

• 0

~

0

•

.0

0

•

0

CD

II

0 •

5000

10000

0

•o

N-

•

0

0

g, 4000

•

DO

15000

•

0

0

0

••

0 0

20000

2000

0

•

0

0

•

0

• ·19

• •

0

0

0

~

0

CIl

0

0

2500

5000

0

•

0

•

7500 10000 12500 15000

ZrCn·cm2)

ZrCn·cm2)

(a)

(b)

Figure 7.36 EIS of galena electrode at different potential with DDTC as the collector (KN03 : 0.1 mollL; DDTC: 10- 4 mollL)

194

7.4

Corrosive Electrochemistry on Surface Redox Reaction of Galena under ... 20000....----------------, 18000 16000

~ 14000 ~

12000 10000 8000 -100

L..--...JL..--JL....-L..--JL..--JL..--JL..--JL..--JL..--J'------'-----''------'

0

100

200

300

400

500

E(mV, SHE)

Figure 7.37 Relationship between polarization resistance and potential of galena with DDTC as the collector

7.4.4 Interactions between Collector and Galena at High pH Figure 7.38 and Fig. 7.39 present the polarization curves of the galena electrode in collector solution at pH= 12 modified by NaOH and lime, respectively. The corrosive electrochemistry parameters are listed in Table 7.6. From Fig. 7.24 to Fig. 27, it has been found that in the absence of collector, the corrosive current 2 and polarization resistance are, respectively 3.05 JlA/cm and 14 ill at pH = 12 2 modified by NaOH; 2.14 JlAIcm and 15 ill at pH = 12 modified by lime. Table 7.6 shows that the corrosive potential and corrosive current decreases and polarization resistance increases in the presence of xanthate and dithiocarbamate at high pH made by NaOH and lime. The corrosive current and polarization 2 resistance are, respectively 1.32 JlA/cm and 17.5 kn at pH = 12 modified by 2 NaOH; 1.29 JlAIcm and 17.4kn at pH= 12 modified by lime. The corrosive 2 current and polarization resistance are, respectively 1.64 JlAIcm and 15.3 ill at 2 pH= 12 modified by NaOH; 1.59 JlAIcm and 15.4 ill at pH= 12 modified by lime. These results show that at high pH modified by NaOH or lime, the formation of collector salt on the galena surface is still possible resulting in the decrease of corrosive current and the increase of polarization resistance of the galena electrode. On the other hand, comparing with natural pH, corrosive potential and corrosive current of the galena electrode decreases very little and polarization resistance has almost not changed, indicating that the interaction between xanthate and/or dithiocarbamate and galena is almost not affected by pH change modified by lime or NaOH. Xanthate and dithiocarbamate may still exhibit good collecting capability on galena in high alkaline media modified by lime or NaOH. It is the basis for selective flotation separation of lead-iron sulphide ore by using xanthate or dithiocarbamate as a collector and lime as a depressant. 195

Chapter 7 CorrosiveElectrochemistry of Oxidation-Reduction of SulphideMinerals 0.4 . - - - - - - - - - - - - - - - - - - , 0.3 --BX - - - Lime+BX -"-NaOH+BX

0.2

0.1

W

=r: 0.0

r:/'l

>='

~-0.1

-0.2 -0.3 -0.4 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 IgI L.-.-----'--_--'--_"--------'-_-'--_....L..----''-------'-------'

Figure 7.38 Polarizationcurves of galena electrodewith xanthate as the collector in differentpH media (KN03 : 0.1 mollL; xanthate: 10- 4 mol/L; unit of I: A/cm2)

0.3

--DDTC ......... Lime+DDTC : -·-NaOH+DDTC .../ /

0.2

0.1

c.u

. . <"1)

0.0

.....

~ -0.1 -;;:

I

:~;.:_. ~----

~ -0.2

-0.3 -0.4 -0.5 -9.0

L.-.--'-------'------1_....L------L.-----...L.._..L.----'-----L.---'

-8.0

-7.0 -6.0 IgI

-5.0

-4.0

Figure 7.39 Polarization curves of galena electrode with DDTC as the collector in differentpH media (KN0 3 : 0.1 mol/L; DDTC: 10- 4 mol/L; unit of I: A/cm2)

Table 7.6 Corrosive electrochemistry parameters of galena electrode in collector solutionat differentpH (collectorconcentration: 10-4 mol/L) Collectors Xanthate

DDTC

196

pH

Rp(kn)

Ecorr(mV, SHE)

Icorr (JlA/em')

nature

16.3

-68

1.46

12(NaOH)

17.5

-77

1.32

12(lime)

17.4

-78

1.29

nature

15.2

-71

1.67

12(NaOH)

15.3

-74

1.64

12(lime)

15.4

-76

1.59

Corrosive Electrochemistry on Surface Redox Reaction of Sphalerite in Different Media

7.5

7.5

Corrosive Electrochemistry on Surface Redox Reaction of Sphalerite in Different Media

7.5.1

Influence of Different pH Media on Sphalerite Oxidation

Because sphalerite is a poor semiconductor, the sphalerite electrode is made of carbon and mineral powder. The corrosive electrochemistry test of the sphalerite electrode is only carried out by electrode polarization. Figure 7.40 is the polarization curves of the sphalerite combination electrode in different pH media. Corrosive electrochemistry parameters are listed Table 7.7. It follows that the corrosive potential and current of sphalerite electrode are, respectively, 42 mV and 0.13 J.1A/cm2 at natural pH, 34 mV and 0.1 J.1A/cm2 at pH= 12 modified by 2 NaOH, and -12 mV and 0.03 JlA/cm at pH= 12 modified by lime. Comparing with natural pH condition, the corrosive potential of sphalerite electrode moves towards negatively and corrosive current decreases when in high pH condition modified by NaOH or Ca(OH)2, indicating the formation of new surface products like hydroxyl species, resulting in the increase of polarization resistance from 48.6 kQ at natural pH to 49.2 ill at pH= 12 modified by NaOH and 68.4 ill at pH modified by lime. The change of corrosive potential and current is greater at pH= 12 modified by Ca(OH)2 than that by NaOH, indicating that Ca(OH)2 has stronger inhibiting function on the sphalerite than NaOH and more surface hydrophilic species will be formed in the lime media, which may produce more depression on sphalerite. Besides, the polarization resistance of sphalerite electrode is the highest among lead-zinc-iron sulphide minerals, illustrating its poor conductivity of ZnS. 0.4r------------------, 0.3 0.2

ZnS - - - ZnS+lime - - ZnS+ NaOH

GJ 0.1 :r: ~ 0.0

~ -0.1

---

-0.2 -0.3

-8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 IgI

Figure 7.40 Polarization curves of sphalerite combination electrode in different pH media (KN03 : 0.1 mol/L; NaOH: 2 x 10- 4 mollL; unit of I: A/cm 2)

197

Chapter 7 Table 7.7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals Corrosive electrochemistry parameters of sphalerite at different pH

I corr (J.1AIcm

Media

pH

Rp(kn)

Ecorr (mV, SHE)

Water

7

48.6

42

0.13

Lime

12

68.4

-12

0.03

NaOH

12

49.2

34

0.10

2 )

NaOH concentration: O.0002mol/L.

7.5.2 Inhibiting Corrosive Mechanism of Collector on Sphalerite Electrode Figure 7.41 is the polarization curves of sphalerite-carbon combination electrode in different collector solution at natural pH. The corrosive electrochemistry parameters are listed in Table 7.8. These results show that xanthate and dithiocarbamatehave distinctly different effects on sphalerite.The corrosive potential 2 and current of sphalerite electrode are, respectively, 42 mV and 0.13 J.1A/cm at 2 natural pH in the absence of collector, - 7 mV and 0.01 J.1A/cm in the presence of 2 xanthate, and 32 mV and 0.12 JlAlcm in the presence of dithiocarbamate. The corrosive potential and current decrease sharply with xanthate as a collector, indicating that the electrode surface has been totally covered by the collector film from the electrode reaction. Xanthate has big inhibiting corrosive efficiency and stronger action on sphalerite. However, the corrosive potential and current of sphalerite electrode have small change with dithiocarbamate as a collector, indicating that DDTC exhibits a weak action on sphalerite. 0.4 . - - - - - - - - - - - - - - - - - - - - ,

0.3 0.2

-ZnS ---ZnS+BX --ZnS+DDTC

Gi' 0.1

::cir: ?

0.0

~ -0.1 -0.2 -0.3

-8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 19I

Figure 7.41 Polarization curves of sphalerite combination electrode with different collectors at natural pH (KN03 : 0.1 mol/L; collector: 10- 4 mol/L; unit of I: A/cm 2)

198

7.5

Corrosive Electrochemistry on Surface Redox Reaction of Sphalerite in Different Media

Table 7.8

Corrosive electrochemistry parameters of ZnS with different collectors

Collectors

1](%)

Rp(ldl)

Ecorr(m~ SHE)

I corr (~cm2)

No collector

7

48.6

42

0.13

Xanthate

32.02

71.5

-7

0.01

DDTC

5.7

52.3

32

0.12

NaOH: 2 X

10- 4

mol/L.

Figures 7.42 and 7.43 are the polarization curves of sphalerite electrode in xanthate and DDTC solution at different pH media. It is obvious that xanthate 0.3

--- BX+NaOH -BX - -BX+lime

0.2 0.1

GJ

:I:

~ 0.0

~-0.1 -0.2

-0.3 -9

-8

-7

-6

-5

-4

IgI

Figure 7.42 Polarization curves of sphalerite combination in xanthate solution at 2) different pH media (KN03 : 0.1 mollL; xanthate: 10- 4 mollL; unit of I: A/cm 0.4~-----------------'

0.3 0.2

$

- - - DDTC+NaOH - - DDTC+lime -DDTC

1/

I

I

I

I

I

II

//

0.1

'/// ~/

~ 0.0 ;;:

~~~~!=iiij;~"'~-"'"

tir' -0.1 -0.2

-0.3 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 IgI

Figure 7.43 Polarization curves of sphalerite combination electrode in dithiocar4 bamate solution at different pH media (KN03 : 0.1 mollL; DDTC: 10- mollL; unit 2) of I: A/cm 199

Chapter 7

Corrosive Electrochemistry of Oxidation-Reduction of Sulphide Minerals

and DDTC also have different effects on sphalerite oxidation at different pH media. When using NaOH and lime as pH modifiers, the corrosive potential and current of the sphalerite electrode decrease in xanthate solution. Xanthate may still have stronger action and certain collecting capacity on sphalerite. However, the corrosive potential and the current of the sphalerite electrode change little in DDTC solution. Dithiocarbamate has not collecting effect on sphalerite at pH= 12 by using NaOH and lime as pH modifiers, which provides the way for flotation separation of galena from sphalerite using dithiocarbamate as the collector.

200

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

Abstract This chapter introduces the changes of electrochemical behavior of sulphide minerals leading by mechnical force exerted in the process of grinding, which is called mechano-electrochemical behavior. To investigate it, two new equipments are applied. The configuration of them is illustrated. And the mechano-electrochemical behavior of pyrite and galena is investigated in this chapter. It is found that after grinding the corrosion and oxidation of these mineral surface are inhibited and produce a reduction atmosphere, which may be useful for the interaction between galena and xanthate to form collector salt and not suitable for the formation of dixanthogen. Meanwhile, surface changes of sulphide minerals under mechanical force are also investigated with Nissan mineral phase micro-camera for observing the surface top graph. And the fact turns out to be that among three sulphide minerals the pyrite erodes the fastest and its eroding current is the largest which is consistent with the previous one. Keywords

mechano-electrochemical behavior; pyrite; sphalerite; galena

Grinding is essential for the liberation of sulphide minerals in order to achieve effective flotation. The grinding process, however, may also have a various effects on the flotation separation because of the galvanic interactions among the grinding media and the different minerals due to the high redox activity of sulphide mineral and iron media as well as thio-reagents. Rey and Formanek (1960) investigated the effect of the grinding media on the flotation of sulphide minerals and showed that sphalerite has natural floatability after grinding with ceramic mill, while has no similar floatability after grinding with steel mill in alkaline pulp. Thornton (1973) reported that the steel media result in the decrease of the floatability of galena, but the floatability of galena had been greatly improved when ground in pyrite media mill. Galvanic cells are set up by redox reactions taking place due to the difference in their rest potential of various species in the grinding system. One of higher potential acts as a cathode, while that of lower potential as an anode. In the grinding-flotation circuit of sulphide ores, galvanic interactions exist between mineral and mineral as well as minerals and grinding media. The galvanic reactions are also governed by the mixed potential principle. The interfacial galvanic interactions of sulphide minerals in the grinding system are affected by hitting or erasing force resulting in the formation of

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

crystal lack, dislocation and the nascent surface. This may be called as mechanical electrochemistry behavior, which results from mechanical and electrochemical process, affecting the semiconductor character of sulphide minerals and its electrochemical behavior. The change of the electrochemical behaviors of sulphide minerals in grinding system will be discussed in this chapter.

8.1 Experiment Equipment A mechanical electrochemical equipment and corrosive couple equipment were designed in order to study the electrochemical behavior of sulphide mineral surface and galvanic interaction based on the method used by Kneer (1997) as shown in Fig. 8.1 and Fig. 8.2, respectively.

3

8

Figure 8.1 Mechanical electrochemistry measurement equipment I-adjuster of rotation speed; 2-model 273; 3-plank:; 4-high speed motor; 5-opposite electrode; 6-reference electrode; 7-digital pressure gauge; 8-lift platform; 9-medium; IO-resin mattress; II-electrolytic cell; 12-working electrode

Figure 8.2 Corrosive couple equipment I-electrochemistry measurement; 2-rnineral opposite electrode;3-reference electrode; 4-working electrode

202

8.2 Mechano-Electrochemical Behavior of Pyrite in Different Grinding Media

The mechanical electrochemistry equipment is different from conventional electrochemistry equipment, which can exert pressure to the surface of the electrode by adjusting the height of the lifting platform and change the pressure of electrode (see Fig. 8.1). Before the experiment, the electrode surface was

polished, and the grinding media, electrolyte and reagent were added in order.

8.2 Mechano-Electrochemical Behavior of Pyrite in Different Grinding Media Different flexibility distortion will be produced on the surface of material due to partial mechanical force, which induce the change of corrosion potential and material surface state. Figure 8.3 shows the variation of corrosion potential and out circuit current of pyrite at static state with Fe as the opposite electrode at natural pH in the presence of xanthate. Figure 8.3 showed that the corrosive potential and current of pyrite decreases gradually with the increase of grinding time. Pyrite appears as cathode when Fe is the opposite electrode, and corrosion potential is finally stabilized at about -100 mV giving rise to reduction atmosphere. Pyrite surface exhibits cathode current and reduction reaction may occur. In this condition, the corrosion and oxidation of pyrite surface is inhibited. According to Chapter 4 and Chapter 7, the potential range of pyrite interaction with xanthate is 100-300 mV. Therefore, dixanthogen can not be formed on pyrite surface by grinding only with iron media. 0.2~--------------'

16 0.1

G3' 0.0

::t 00

\

8 \

E \

,

~

.-~---~~-----~O

>"

~-0.1

'-1 ---------_

E

,.

o ~

-8

-0.2 -16 -0.3 '--..L....-..L.--~___L..____L_____'_______L___'______L..____L_I.-------I o 100 200 300 400 500 600 t(s)

Figure 8.3 Variation of corrosion potential and out circuit current of pyrite electrode at static state with Fe as opposite electrode (pH = 7, BX: 2 x 10- 4 mol/L)

Figure 8.4 is the variation of potential and the current of pyrite electrode at different pressure with Fe powder as the grinding media at natural pH in the 203

Chapter 8

Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

presence of xanthate. It is obvious that the potential of the pyrite electrode decreases with the increase of mechanical pressure exerted on it, which results in the increase of the grinding action between pyrite electrode and iron media. At about 7 min and the mechanical pressure at 800 g, the potential of the pyrite electrode is only the lowest -0.25 V. The pyrite surface exhibits stronger reduction atmosphere under grinding with iron media. Here, the pyrite surface potential is far lower than the potential for the formation of dixanthogen and floatation of pyrite. Therefore, mechanical pressure due to grinding only in iron media is detrimental to electrochemical action between xanthate and pyrite. 0.20 r - - - - - y - - - . , . - - - . . - - - - - , . . - - - - r - - - - - - - - - , I I I I I I I I I I I I

0.15 0.10

I I I I I

0.05

G3' 0.00

400g

J:

~ -0.05

~ -0.10 -0.15 -0.20 -0.25

-~---~

I

:

:': I'

I I I _~

I

: I I I

~

16 800g

Og

8

:<

E 0 1 0

--~------r---~---

OgI 200g I " I

600g

I I I I I I I I I I I I

I

'"

: I I I

:

L_"'\:_I

I

I

I I

I I

L

-8

I

:~-_

~

~

I I

I I ~

_

-16

-0.30 L..-~_.L_._-...J.-_L...__....L..__.L_._-...J.-_L...___'__~___L._---' o 100 200 300 400 500 600 t(s)

Figure 8.4 Variation of potential and current of pyrite electrode at different mechanical pressure with Fe powder as grinding media (pH = 7, BX: 2 x 10- 4 mol/L)

The variation of corrosion potential and out circuit current of pyrite electrode at static state with sphalerite as the opposite electrode at natural pH in the presence of xanthate is presented in Fig. 8.5. It is the variation of the electrode potential and the current of pyrite electrode under different grinding pressure with sphalerite as the grinding media at natural pH in the presence of xanthate. It can be seen from Fig. 8.5 that pyrite still exhibits the cathodic characteristic when sphalerite is used as the opposite electrode at static state. The corrosion potential of the pyrite electrode decreases at the beginning and is fmally stabilized at about 140 mV. The pyrite electrode has not exhibited obvious cathode current. When sphalerite is used as the grinding media as seen from Fig. 8.6, the potential of pyrite electrode decreases with the increase of the mechanical pressure exerted on it and the grinding time. Pyrite exhibits cathodic characteristic, but the degree of cathode polarization is less than that in Fe grinding media. Corrosion potential of the pyrite electrode reaches to the lowest value about 145 mV at pressure of 800 g and 8 min. Comparing Fig. 8.3 with Fig. 8.5, and Fig. 8.4 with Fig. 8.6, it can be found that the corrosive potential of the pyrite electrode is higher either by using 204

8.2 Mechano-Electrochemical Behaviorof Pyrite in DifferentGrindingMedia

16 0.18 8

G3' ~ 0.15

...... .......

0

;;>

,.<'8 0

.-4

tiS

~

E

-8 0.12 -16

o

100

200

300 I(S)

400

500

600

Figure 8.5 Variation of corrosion potential andout circuit current of pyriteelectrode at static state with sphaleriteas oppositeelectrode(pH = 7, BX: 2 x 10-4 mol/L)

I

0.19

16

I I I I

0.18

I I I

0.17

-..

I

I I I I I I I I

0.15 Og

0.14

o

.....----l _ __

I :

- T ' _ _ .....

____ L

~____

I I I

: 200g: 400g I I

100

200

1

--~

600g 300 I(S)

I I /-

__

L~~_~

I I I I I I I I

I I

8

400

I I I I I I I

800g 500

<'

o ,.E o

-8 Og

-16

600

Figure 8.6 Variation of electrode potential and current of pyrite at different mechanical pressure with sphalerite as grinding media(pH = 7, BX: 2 x 10- 4 mollL)

sphalerite instead of iron powder as opposite electrode or by using sphalerite instead of iron powder as grinding media. When iron powder is used as the opposite electrode or as the grinding media, pyrite surface exhibits strong cathode current. When sphalerite is used as the opposite electrode or as the grinding media, no cathode current occurs on the pyrite surface. These results demonstrate that the strong reduction atmosphere will be induced for pyrite in iron medium grinding, which may be changed to the less reduction atmosphere when coexisted with sphalerite. For the coupling electrodes of sphalerite and pyrite, xanthate may show weak interaction with sphalerite at this potential and no action with pyrite. 205

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

The variation of the corrosion potential and the current of pyrite at static state with galena as the opposite electrode and at different pressure with galena as the grinding media are, respectively, shown in Fig. 8.7 and Fig. 8.8. It can be seen from Fig. 8.7 that the corrosion potential and the current of the pyrite electrode demonstrate the tendency of going down when galena is used as the opposite electrode. The corrosion potential of the pyrite electrode is stabilized at about 70 mV and the surface exhibits cathode current showing reduction atmosphere. Fig. 8.8 shows that with galena as the grinding media, the corrosion potential of the pyrite electrode changes with the exerting pressure, which reaches about 70 mV in 6 min. The cathode current occurs on the pyrite surface. After 8 min, the corrosion potential of the pyrite electrode increases to 150 mV and a small

16

0.18

8 ~

<'

0.12 ,

- ~~ --.:.-...:.-...:.::.::.:..: :..: :.:-: I -----

~

~

E

b

0

~

E

~ 0.06

-8 -16

o

100

200

300

t(s)

400

500

600

Figure 8.7 Variation of corrosion potential and current of pyrite electrode at static state with galena as opposite electrode (pH = 7, BX: 2 x 10- 4 mollL) 0.20 . . . - - - - - T - - - - - . - - - - r - - - - - - r - - - - r - - - - - - , I

rJJ

~ ~

0.14

I

I

1

:800g :

I I I 1

1 ___

1 1

~ ~

0.12 0.10 Og 0.08

I \

16

1

1

0.16

~

I I

1

0.18

I

1 1 I

1 I I

I

1 1 1__

I - - _1-

I __ L

I I

I

: ,-~-

I

I 1

8

<'E

o b

-1_-...1

I I I

I I 1

I I

:200g

:400g:

I I

I I

I 1

:

:

:600g:

1

I

-8 -16

0.06 L...--L--J.L--J_.....L.L---L...-'---L----L....L..-.....L-L---'-_~......L..__---' o 100 200 300 400 500 600 t(s)

Figure 8.8 Variation of potential and current of pyrite electrode at different mechanical pressure with galena as grinding media (pH = 7, BX: 2 x lO-4 mollL)

206

8.2

Mechano-Electrochemical Behavior of Pyrite in Different Grinding Media

anode current occurs on the pyrite surface. These results suggest that for the coupling electrodes of galena and pyrite, the potential of the pyrite electrode is lower than the formation potential of dixanthogen, so pyrite can not interact with xanthate. Although with the increase of grinding time, the electrode potential of pyrite may increase to some extent, it is still outside of the range of the potential of action with xanthate. The results from Fig. 8.3 to Fig. 8.8 demonstrate that the potential of the pyrite electrode always decreases when galena, sphalerite and iron are used as the grinding media. The reason is that a corrosion couple has been established when pyrite contacted with other media. The principle may be schematically presented in Fig. 8.9. It shows that the rest potential of pyrite, sphalerite, galena and iron are 180, 50, -50, -440 mY, respectively. Due to the high static potential of pyrite, it becomes the cathode and other media become the anode when pyrite contacts with other media. During the process of reaction, the surface static potential of pyrite reaches towards anode polarization and other media reach towards cathode polarization. When cathode potential is equal to anode potential the reaction achieves equilibrium, accounting for the decreasing tendency of the potential of pyrite electrode when using galena, sphalerite and iron powder as the opposite electrode, which is more in the grinding atmosphere.

O.18V Pyrite

El -----------I E2 -----------

----

O.05V

I

--~-~

_--+----.J

I

E3

I

- - -/T - - - I - - - - - - - - /

--+--'

I

I

I

I

I

I

/

Fe

Figure 8.9 media

Illustration of polarization of corrosion couple between minerals and

In flotation practice, the reducing environment produced in the grinding system may be useful for the interaction between galena and xanthate to form collector salt and not suitable for the formation of dixanthogen. It may promote the selective flotation separation of galena from pyrite. Figure 8.10 is the variation of the potential and the current of the pyrite electrode with pyrite as the grinding media at different pressure at natural pH in the presence of xanthate. The results show that the corrosion potential decreases with the increase of mechanical pressure. The range of potential change is in 170 - 190 mV. This change may arise from the formation of nascent surface

207

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

under grinding force. That is to say, the newly exposed pyrite surface may show lower potential due to grinding. Pyrite electrode surface appears as anode current, which changes little with the time and the exerting pressure. 0.195

,

,,

0.185

,

0.175

CfJ

0.170

:r::

: 1:

'- T -' -,-- -

0.155

1 -1-

____ .L ____ ,_____ 1. ___

I I I I

'> ~ 0.165 0.160

8

I I

0.180 ~

16

I I

0.190

, I I

Og

200g:

,

I I

I I

1 1 I I I

<'E

.r-:: _

1"0

I I I

~

1 1

-8

I I

400g : 600g: 800g I I I

Og

1

-16

I I

0.150 '--.....L-~---'---'-L--.....L--'-'----L......L.-L-o 100 200 300 400 t(s)

...............- " ' " - - - - L . . - - - '

500

600

Figure 8.10 Variation of potential and current of pyrite electrode at different mechanical pressure with pyrite as the grinding media (pH = 7, BX: 2 x 10- 4 mol/L)

8.3 Mechano-Electrochemistry Process of Galena in Different Grinding Media Figure 8.11 gives the variation of the corrosion potential and the out circuit current of the galena electrode at static state with Fe as the opposite electrode in the presence of butyl xanthate at natural pH. Fig. 8.12 is the variation of the potential and the current of the galena electrode at different pressure with Fe powder as the grinding media in the presence of butyl xanthate at natural pH. It can be seen from Fig. 8.11 that the potential of the galena electrode reduces and reaches to a stable value of about -160 mV after 3 min. Galena appears as cathode when Fe is the opposite electrode and its surface exhibits cathode current, in which galena surface is protected and surface corrosion is inhibited. Figure 8.12 exhibits that when Fe powder is used as the grinding media, the corrosion potential of galena decreases with the increase of time and the exerting pressure. When the exerting pressure is 800 g, the corrosion potential of galena decreases to about -190 m~ which is lower than that when Fe is used as the opposite electrode, and the galena surface appears to have higher cathodic current. It indicates that the mechanical force by grinding may promote the reduction atmosphere. When the out exerting pressure is removed in about 500 s, the corrosion potential of galena reconciles to about -50 mV and the cathodic current of galena decreases and even a small anodic current occurs. These results suggest 208

8.3

Mechano-Electrochemistry Process of Galena in Different Grinding Media

that the potential range after grinding in Fe media is suitable for the formation of lead xanthate and hence for the flotation of galena, which is usually corresponding to the initial potential of galena listed in Chapter 4. 0.05.--------------------,8

6

o

4

-0.05

~

2

~ -0.10 \

~

E -,-,---------------------------- 0 1 ~ -0.15 / 0 -2 ~ ~

-0.20

-4

-0.25 -0.30

-6 100

0

200

300 t(s)

400

-8 600

500

Figure 8.11 Variation of corrosion potential and current of galena electrode at static state with Fe as opposite electrode (pH = 7, BX: 2 x 10- 4 mol/L) 0.05...----..--------,....---------.---..--------,------, 8 I

-0.05

I I I I I I I

-0.10

:

o

~ ~

4

I

2

I

r/l

~

6

---"

I

I

I

I

I

I

0

---~~~~--i-----r---i-----i-

-0.15

I ........ +-

-2

II'~I----I-----rI I I I

-0.20 -0.25 -0.30

Og

" - --

I I I

.L.......L.-

100

-

L.--L--

200

I I I - - L- J . --

300 t(s)

I 0

~

-4

200g: 400g : 600g: 800g I I I

0

I I I

~ E ~

Og

-6

I I I .L..--L...-----'-----'--

400

500

-

-8 600 ---'

Figure 8.12 Variation of potential and current of galena electrode at different mechanical pressure with Fe powder as grinding media (pH = 7, BX: 2x 10-4 mol/L)

In the presence of butyl xanthate and at natural pH, the variation of the corrosion potential and the out circuit current of galena electrode at static state with sphalerite as the opposite electrode is presented in Fig. 8.13. The variation of the potential and current of the galena electrode at different pressure with sphalerite as the grinding media is presented in Fig. 8.14. Figures 8.13 and 8.14

209

Chapter 8 Mechano-Electrochemical Behaviorof Flotationof SulphideMinerals

show that for the coupling electrodes of galena and sphalerite, the corrosive potential of the galena electrode reaches a stable value of about -25 mV after 3 min. Galena is anode and its surface exhibits obvious anode current. The corrosion on the surface of galena is intensified when sphalerite is the opposite electrode. The change of the corrosion potential of galena is waved when sphalerite is used as grinding media. The potential decreases when grinding pressure is put on the galena surface at the beginning for each loading and then potential increases gradually when friction achieves stability. There may be existing two kinds of 0r-------------------,8

E

6 4

~

::r:

"",--,....

2

..... - - - - - - - - - - -

~ -0.10 -------------------------------- 0 ~

-2

~ E

'1 o

~

-4

-0.15

-6 -0.20 0

100

200

300 t(s)

400

500

8 600-

Figure 8.13 Variation of corrosion potential andoutcircuit current of galena electrode at static state with sphalerite as oppositeelectrode (pH = 7, BX: 2 x 10- 4 mol/L) O,...-----r----.,----or-----.----,.-----, 8

6

4

-0.05 1

1

I

,

ClJ

~

~

>

2

1

I

:

:

1

_~, ,,"'1- -'...... - ..... ."........1\ ,-j -, I I -0.1 0 - - - - - - - ~ V L - - - - ~~;-' - -,- - "\.- i':' ~ - - - - I

,

I

1

200g : 400g

600g: 800g

~

1 1 1

-0.15

Og

00

I 1 I

1

,

1

,

200

-2

, , ,

1 1 1

300 t(s)

400

0

Og

~ E

'1 0

~

-4 -6

500

Figure 8.14 Variation of electrode potential and current of galena at different mechanical pressure with sphalerite as grinding media(pH = 7, BX: 2 x 10-4 mol/L)

210

8.3

Mechano-Electrochemistry Process of Galena in Different Grinding Media

factors which influence corrosion potential. One is that nascent galena surface is anode, the other is that the grinding media (sphalerite) is cathode. Nascent surface increases and the potential decreases when pressure is put on galena surface. When the increase of nascent surface achieves equilibrium, the coupling action between grinding media and galena occupies dominant position and potential starts to increase. These results suggest that in lead-zinc ore the potential of mineral changes frequently due to grinding. However, it can be found from Fig. 8.14 that the change of galena potential is around -50 mY, which may produce little effect on the interactions between the collector and galena. Figure 8.15 shows the variation of the corrosion potential and the out circuit current of the galena electrode at static state with pyrite as the opposite electrode in the presence of butyl xanthate at natural pH. Figure 8.16 is the variation of the electrode potential and the current of the galena electrode at different pressure with pyrite as grinding media. It follows that galena appears as anode with an increase of potential and its surface exhibits obvious anode current when pyrite is used as the opposite electrode, where the anode reaction between galena surface and xanthate may be speeded up. When pyrite is used as grinding media, the same phenomena of using sphalerite as the grinding media can be observed. The potential of galena electrode changes with the exerting force and time, which decreases firstly and then increases with the grinding time increasing. This is also because two factors mutually affect corrosion potential. Nascent galena surface increases due to grinding and potential decreases when pressure is put on galena surface. When the increase of nascent surface achieves equilibrium, the coupling action between the pyrite media and the galena electrode results in the increase of potential. The change range of galena potential is from -70 mV to -40 mV. When the out exerting pressure is removed at about 500 s, the corrosion potential 0.00,....------------------, 16 -0.03

---- --------1

8

-8 -0.09

-16

o

100

200

300 400 t(s)

500

600

Figure 8.15 Variation of corrosion potential and out circuit current of galena electrode at static state withpyrite as opposite electrode (pH= 7, BX: 2 xl 0-4 mol/L)

211

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals 0.00 r---....-----r-----.----~---r------, 8 I I I I I

6 4

1

-0.03

I I I

w:

';>

~

"" -

- - - - -

--+-_/

:? 8 o ,2

1

:c r:/'J

I

~-

- - _(.1

.......

_

_1

o

.-;

-0.06

Og

200g

I I I I I I

600g

-2 ~

-4

800g:

-0.09

Og

-6

1

I

o

-8 600

\......----J--""---I-_I--..l....J----..J-'""""----'-----J-----L.L-_'---'---J----..J-----'

100

200

300 t(s)

00

500

Figure 8.16 Variation of electrode potential and current of galena electrode at different mechanical pressure with pyrite as grinding media (pH = 7, BX: 2 x IO-4 mollL)

of galena electrode reconciles to about - 35mV and a small anodic current occurs, suggesting that the potential range after grinding in sphalerite media is suitable for the formation of lead xanthate and hence for the flotation of galena. Figure 8.17 reflects the variation of the electrode potential and the current of galena electrode at different pressure with galena as grinding media in the presence of butyl xanthate at natural pH. The results show that corrosion potential decreases with the increase of mechanical pressure. This is because the nascent galena surface has strong anode character. More and more nascent surface may be produced due to grinding giving rise to the descending of potential. -0.00

8 6

-0.05

4

I I 1

2

1

GJ :c

I I

I

ir:

-;;: -0.10 - -=-_-:1..::, -..:. -:-

~--~----~

I I

~

-0.15

-0.20

Og

o

I I

/-,--...... I I -

-J~ ::-: 0 ,,;.~ I

-2

I

I

1

I

I I

I I

I I

I I

: 600g : 800g: Og

-4

1 1

I I

I I

I I

I I

I I

I I

I I

-6

200g: 400g

100

--

200

300 t(s)

400

500

:?

,.E 0

~

-8 600

Figure 8.17 Variation of electrode potential and current of galena at different mechanical pressure with PbS as grinding media (pH = 7, BX: 2 x·l0- 4 mollL)

212

8.4

Influence of Mechanical Force on the Electrode Process between Xanthate and ...

Figure 8.18 illustrates the polarization of the corrosion couple between galena and other minerals. Galena has low static potential in the three sulphide minerals. When it contacts with other media, they form the corrosion cell. Galena appears anode and cathode polarization occurs. When galena contacts with Fe medium, which has lower potential than galena, the result is contrary (see Fig. 8.9).

O.18V

Pyrite O.05V

\

\

\

\

~, - - - - - - §! - - -

----

I

\

I

ZnS \ / E2 -O.05V - - ~ - - - - - - - - - - - - - PbS

-0.44V

\

E3

---~------------/

\

Fe

Figure 8.18 Schematical diagram of polarization of corrosion couple between galena and other minerals

8.4 Influence of Mechanical Force on the Electrode Process between Xanthate' and Sulphide Minerals As indicated by the previous research, the electrode process between collector and sulphide mineral on its surface is closely related to the surface activity of the mineral, which is greatly influenced by grinding in flotation process. It is very important to investigate the influence of mechanical force on the mineral electrode process so as to reveal the surface process of the sulphide minerals. Figure 8.19 is the Tafel curves of the pyrite electrode in xanthate solution under the conditions of the mechanical power and non-mechanical powers. From Fig. 8.19 it can be seen that under non-mechanical powers the Tafel slope of the pyrite electrode is 120 mV and the IgIo is -5.45, indicating that the reaction FeS2+X- = FeS2- X(ad) + e- is the crucial step, i.e. the adsorption of xanthate is the controlled step. Under the action of mechanical power, the Tafel slope of the pyrite electrode is 40 mV and the IgIo is -5.35. The reaction FeS2 - X(ad)+X- = FeS2-X2 is the crucial step, i.e. the formation of dixanthogen due to the adsorbed xanthate is the controlled step. In this case the mechanical power through the increasing the number of the active points on the surface leads to the rise of the reaction rates of FeS2+X- = FeS2 -X(ad) + e- and affects the equilibrium constant K. 213

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals 0.35 r - - - - - - - - - - - - - - - - - ,

0.30 ~ ~

Mechanical action ....... No mechanical action

0.25

:t ir:

';> 0.20

~

0.15 0.10 -5.8 -5.6 -5.4 -5.2 -5.0 -4.8 -4.6 -4.4 -4.2 -4.0 IgI

Figure 8.19 Polarization of pyrite electrode under different mechanical pressure (unit of I: AIcnr')

According to the following equation 2F IgI = Ig(2FkBK 2 ) + 2lgcx - - - E 2.3RT

The equilibrium constant K influences IgIo value and hence affects the Tafel slope and the electrode process. The Tafel curves of the galena electrode in xanthate solution under the conditions of the mechanical power and non-mechanical powers are given in Fig. 8.20. It follows from Fig. 8.20 that IgIo of the anode action on the surface of galena raises from -6.5 to -5.9 with the increase of Tafel slope from -40 mV to - 28 mV under the mechanical action. It shows that the reaction is favored in dynamics due to the grinding. 0.20

Mechanical action ....... No mechanical action

0.15 0.10

G3' :I:

rfJ

';>

0.05

ttr

0.00 -0.05 -0.10

....... -6.8

-6.4

....

.' .. .. ' ..

'

.'

.' .'

.. .. .' '

'

.'

.'

'

-6.0

-5.6

-5.2

-4.8

-4.4 -4.0

19l

Figure 8.20 Polarization of galena electrode under different mechanical pressure (unit of I: Alcm2) 214

8.5 Surface Change of Sulphide Minerals underMechanical Force

8.5

Surface Change of Sulphide Minerals under Mechanical Force

Different mechanical force and grinding media induce the change of surface character, which causes the change of potential of mineral surfaces. The surface changing can be investigated by observing the surface top graph which results from mechanical rubbing of the mineral surfaces with the media of different sizes. Iron media sizes are as follows: for coarse grinding (+ 30- - 70 urn), for middling grinding (+ 10-- 30 urn), for fine grinding (- 10 urn). The pictures of the top graph are taken by Nissan Mineral Phase Micro-camera with the amplification of 1000 times.

8.5.1

Surface Change of the Pyrite under Mechanical Force

The surface appearances of the pyrite ground by the Fe media of different size are presented in Fig. 8.21 to Fig. 8.24. It can be seen from Fig. 8.21 that the original surface of pyrite is smooth and undamaged. When the mechanical action is imposed to pyrite, its surface turns uneven due to the friction. Since there are height differences on the uneven surface area, the whole viewing area can not be concentrated at a focus and some parts get obscure which is most obvious in the case of the coarse grinding. At this time, the surface is the most uneven. When pyrite is abraded in the middling size grinding condition, the specific surface area increases further and some colored species exist in the viewing area, demonstrating that the minerals have experienced some reactions. Under the fine grinding media, pyrite surface appears rougher and more colorful species are observed, indicating more intensive surface reaction.

Figure 8.21 Original pyrite surface 215

Chapter 8

Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

Figure8.22 Pyrite surface in coarse particle media

Figure8.23 Pyrite surface in middling particle media

Figure8.24 Pyrite surface in fine particle media 216

8.5

8.5.2

Surface Change of Sulphide Minerals under Mechanical Force

Surface Change of Sphalerite in Mechanical Force

Figures 8.25, 8.26, 8.27 and 8.28 are surface appearance of sphalerite at original state, and ground by coarse media, middling media and fine media, respectively. The figures above show that sphalerite has similar changes to those of pyrite under the four different conditions except that the colorization of sphalerite surface is not as evident as that of pyrite under the fine grinding condition. It indicates that there may be more active species to be formed on the surface of pyrite than sphalerite. This is consistent with the findings from the previous researches that among the three sulphide minerals the pyrite erodes are the fastest and its eroding current is the largest.

Figure 8.25

Figure 8.26

Original sphalerite surface

Sphalerite surface in coarse particle media

217

Chapter 8 Mechano-Electrochemical Behavior of Flotation of Sulphide Minerals

Figure 8.27

Sphalerite surface in middling particle media

Figure 8.28

218

Sphalerite surface in fine particle media

Chapter 9 Molecular Orbital and Energy Band Theory Approach of Electrochemical Flotation of Sulphide Minerals

Abstract In the light of quantum chemistry, sulphide minerals and the interaction of them with reagents are investigated in this chapter. With the density functional theory pyrite is first researched including its bulk properties about energy band and frontier orbitals and the property ofFeS2 (100) surface. It is found that surface Fe and S atom have more ionic properties. And both Fe2+ and S2- have high electrochemistry reduction activity, as is the base of oxygen adsorption. Thereafter, the configuration of oxygen adsorping on the surface of pyrite is studied. And from the viewpoint of adsorption energy, the parallel form of oxygen adsorption is in preference to the perpendicular form. Similarly, some properties of ganlena are investigated. Furthermore, the activation of sphalerite is also researched. And the conclusion is that the forbidden gap of doped ZnS will decrease and the electrochemistry activity of ZnS is in return enhanced. The reaction between reagents and minerals is also discussed in the terms of molecular orbital and energy band, which provides a new method to investigate the mechanism of flotation.

Keywords

molecular orbital; energy band; density functional theory(DFT)

It is well known that the flotation of sulphides is an electrochemical process, and the adsorption of collectors on the surface of mineral results from the electrons transfer between the mineral surface and the oxidation-reduction composition in the pulp. According to the electrochemical principles and the semiconductor energy band theories, we know that this kind of electron transfer process is decided by electronic structure of the mineral surface and oxidation-reduction activity of the reagent. In this chapter, the flotation mechanism and electron transferring mechanism between a mineral and a reagent will be discussed in the light of the quantum chemistry calculation and the density function theory (DFT) as tools.

9.1 Qualitative Molecular Orbital and Band Models The frontier molecular orbital, including the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital), plays a key

Chapter 9 Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

important role in a chemical reaction. When a chemical reaction takes place, the electron transfers from the HOMO of a molecule to LUMO of another one, in which the HOMO and LUMO must be o or 1t symmetrical and the energy of both orbitals is close. When a group of atoms is brought together to form a solid, the individual electronic energy levels of the separate atoms overlap to form bands of closely spaced energy levels. The lower bands are completely filled with electrons, which are called filled bands or valence bands. The electrons in a filled band cannot normally move, but they can be forced up into unoccupied band by applying large amount of energy. The completely unoccupied or empty band is called an empty band, which becomes the conduction band when electrons are excited into the band from the filled band. The top of valence band and the bottom of the conduction band are separated by a forbidden energy gap or band gap. If the separation is large, i.e. an energy of 5 - 10 eV is required to promote an electron from the valence band to the conduction band then the solid would be an insulator. No forbidden energy gap exists for metal solid. If the forbidden energy gap is considerably smaller (perhaps 0.2 - 0.3 eV) then the material would be an intrinsic semiconductor, which can be divided into p-type which contains "holes" through which conduction may occur and n-type which contains electrons through which conduction may occur. The holes and electrons through which conduction occurs are called the conduction carrier of semiconductor. The conduction electrons are in the bottom of the conduction band and the conduction holes are in the top of valence band. Most sulphide minerals have intrinsic semiconductor structure. The interactions across the mineral-solution interface in the single presence or coexistence of oxygen, collector, HS- etc. may take place by two paths, one of which is that the electrons are transferred from the bottom of the conduction band into the LUMO of molecule of the reactants and the other is from HOMO into the holes of the top of valence band, in both of which energy must be close.

9.2

Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface

It has been shown in the previous chapters that the product of the electrochemical reactions in the sulphide flotation system is determined by the mixed potential of the flotation pulp. The value of the potential is dependent on the equilibrium of anodic and cathodic process existing in the pulp. In general, the most important cathodic reaction existing in the pulp is the oxygen reduction. To rewrite Eq. (1-1) as the following: (9-1) If the access of O2 is limited, the cathodic reaction and hence the coupled

220

9.2

Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface

electrochemical reactions will be confined. Therefore, the oxygen reduction on the sulphide mineral surface plays a pivotal role in the flotation of the sulphide minerals. Oxygen reduction has been extensively studied in catalyze, fuel cell and corrosion (Pattabi et al., 2001; King et al., 1995). The reaction was often described as a multiple-step process, which is illustrated in Eq. (9-2): adsorption)

0*

~H

2~

0

22

~ 20H-

(9-2)

It is reported that oxygen reduction is an electro-catalytic process (Fierro, et al., 1988; Gupta, et al., 1989) and that the rate is largely dependent on the properties of the surface. Chung's works (1998) showed that different surfaces have different affinity to oxygen, which has an important effect on oxygen reduction. Ahlberg and Broo (1996a,b,c) studied the oxygen reduction on pyrite and galena with a rotating ring disc electrode. Their results suggest that the first electron transfer is the rate-determining step for the oxygen reduction at both galena and pyrite and hydrogen peroxide is found to be an intermediate. While galena is a poor catalyst for oxygen reduction, with minor formation of hydrogen peroxide, pyrite is a relatively good catalyst. This kind of difference may lead to different flotation behaviors. One approach to a better understanding of the properties of the solid surface is to model the electron structure with quantum mechanical methods, which is a useful complement to experimental techniques. It allows direct observation of atomic-scale phenomena in complete isolation, which cannot be achieved in current experimental studies. A lot of work (Opahle et al., 2000; Andrew et al., 2002a,b; Muscat et al., 2002; Edelbro et al., 2003) has been performed on bulk and surface properties of FeS2 using various kinds of density functional theory. These works have shown that such methods are capable of producing calculated bulk properties such as lattice constants which agree well with experiment, and has provided a reference for the study of pyrite surfaces.

9.2.1 Computation Methods The geometry optimization of FeS2-02 and electric-structure-calculation of pyrite and its surface presented here is performed using ab intio simulation program Cambridge serial total energy package (CASTEP) through CERIUS2 graphical user interface. The wave functions are expended in a plane wave basis set, and the effective potential of ions is described by ultrasoft pseudo potential. The generalized gradient approximation (GGA)-PW91, and local gradient-corrected exchangecorrelation functional (LDA)-CAPZ are used for the exchange-correlation functional. 221

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

In order to find a reasonable configuration for our calculation, we take test calculation to optimize the bulk structure of pyrite with GGA and LDA exchange-correlation functional. In the calculation, the plane wave cutoff energy set is 280 eV and the key point set is 4 x 4 x 4, the convergence tolerances set is 10- 5 eV/atom. The optimized cell parameter of the two methods is 0.5415 nm and 0.5425 nm respectively, which is in good agreement with the experiment data (0.5417 nm) reported. It indicates that this configuration is sufficient to satisfy the request of accuracy. The most important FeSz surface is the (100) surface, which is the most common growth surface and is also the perfect cleavage surface. Research from Nesbitt et al. (1998) suggest that the (100) surface of pyrite exhibits good stability and only minimal relaxation from the truncated solid. Therefore, our adsorption calculation is based on FeSz (100) surface and the relaxation of surface is ignored. The FeSz (100) surfaces are modeled using the supercell approximation. Surfaces are cleaved from a GGA optimized crystal structure of pyrite. A vacuum spacing of 1.5 nm is inserted in the z-direction to form a slab and mimic a 2D surface. This has been shown to be sufficient to eliminate the interactions between the mirror images in the z-direction due to the periodic boundary conditions. Surface models used a [1 x 1x 1] crystal unit cell as a surface slab in the supercell. We have also investigated the effect of slab thickness on the calculation result. It shows that three Fe- S layer model can get almost the same result as four Fe- S layers model. Two models of oxygen adsorption are considered, vertical form and parallel form, which are illustrated in Fig.9.1 and Fig. 9.2. For all the cases, the adsorbate/substrate system is optimized by GGA. In optimization, all atoms on the pyrite substrate are fixed; only the 0 atoms are allowed to move. The initial 0- 0 double bond length and the distance between Fe atom and 0 atom are 0.121 nm and 0.196 nm, respectively. To simplify the calculation, the adsorption coverage will not be considered.

(a)

(b)

Figure 9.1 Vertical form of 0 adsorption on FeSz (100) surface (a) Side view; (b) Top view

222

9.2

Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface

00

~

Figure 9.2 Parallel form of 0 adsorption on FeS2 (100) surface (a) Side view; (b) Top view

Theadsorption energies aredefined positive fora stable adsorbate/substrate system: (9-3)

where E Fes,-o, is the energy of the adsorbate/substrate system, Eo, and E Fes, are the energyof the adsorbate and the substrate alone.

9.2.2 Bulk FeS2 Properties GGA property calculation on optimized bulk pyrite has been performed. The calculated band structure and frontier orbital distribution are presented in Fig. 9.3 and Fig. 9.4. The calculated band gap of FeS2 is 0.97 eV, and it is in good line 4

--=:::::::::::,

~

2

- - - - - - --

o -2 -4

> ~ -6

~

----

Sil

~ -8

U..l

- ---

--

~

--

-1 0 - 12 - 14 -16

x

- --

R

~

---- ---G

M

R

k space

Figure 9.3 Calculated band structure of bulk FeS2

223

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

Figure 9.4 The frontier orbital distribution in bulk FeSz I-lowest unoccupied state; 2- highest occupied state

with the experimental data (0.96 eV). The density of states (DOS) diagram for bulk pyrite is shown in Fig. 9.5. These figures show that the Fe (3d) non-bonding orbitals are located at the top of the valence band and extend to just below the Fermi level. Another part of Fe (3d) orbital and antibonding Sp3 orbitals of sulfur contributes to the conduction band (CB). These findings are consistent with the crystal field theory (CFT) description of a metal cation surrounded by ligands in an octahedral array, with unoccupied Fe (3d) eg states immediately above the

I![_- -,L _ _~- -" ':. . .,L. A, .> ._"- - "- '- . L- . L- ~- "- - '- ~- -L" '- - ',_ _Total

1

d:\

I

-20

I

-15

5

-:-10

nl =~ ~ i!__ _ '_- ' - _L.-_-'-_-'- _. . . . ,= ~=' -Lo_=Oo.' _~' _ _"' ' _ _ _ ' '" c:

s

3s

.Q

.~

---<\

R,

I _--L_--'~=-""""'_"--"---'--_...L-_--'-_---'-~::="---"'-" I

-20

-15

-10

-5

I

I

-15

I

-10

~£I.\

-5

0

Energy(eV)

Figure 9.5 Total and partial density of state of bulk FeSz

224

_

I ____'_ _

5

13d

Fe

-20

0

I

__

5

9.2

Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface

fully filled t2g states. The same conclusion has been reported by Andrew Hung et al. (2002). The peaks from -11 eV to -17 eV come from S (3s) state. Below the valence band, there is a mixture of Fe (3d), S (3p) and S (3s) orbital; it is the area of Fe- S bond and S- S bond.

9.2.3 Property of FeSz (100) Surface In bulk FeS2, each Fe atom is coordinated with six S atoms and each S atom is coordinated with three Fe atoms and one S atom. Fe atom has two free electrons and S has 6 free electrons. For S atom, one electron is used to form S- S dimmer and the other 5 electrons are used to form Fe- S bond. For Fe atom, all the free electrons are shared by Fe- S bond. That is to say the electron contribution of Fe atom to each Fe- S bond is 2/6 and that of S atom is 10/6. When the crystal is cleaved from (l00) directions, only Fe- S bonds is broken (see Fig. 9.6). Then the 6-fold Fe atom becomes 5-fold Fe atom and 4-fold S atom becomes 3-fold S atom. Thus, it will produce dangling bonds and surface states.

I

-==:::::::,

I

I I I I

1/

I====I

I

U

I I I I

--Figure 9.6

Schematic cleavage ofFeS2 from (100) directions

Figure 9.7 presents the DOS distribution of FeS2 (l00) surface. By comparing the PDOS (partial density of state) on the surface with the PDOS from the bulk, a slight narrowing and an upward shift of the Fe (3d) band can be seen because the surface Fe atom loses one neighbor S atom. However, the most important effects

225

Chapter9 Molecular Orbitaland Energy Band TheoryApproach of Electrochemical ...

of these coordinative unsaturations appear with the S (3p) band, which shows a few peaks around the Fermi level and its bottom now starts only at -6 eY. These unsaturated S (3p) bands playa crucial role in frontier orbitals. 6

S on surface

4 2

.......

......

01.-_---L----.:...._-::-_~...:.&___L__

:> ~ ~

e

6

Q3

2

-10

rJJ

_ _ L _ . L _ _ . . . . . L . . . __

-5

___1_~_L__~__L______I

: : 3d

Fe on surface

"0 4 Q)

o

_

o r-------------------------. -15

-20

OOI.-_---L-_--'-_ _ -20 -15

....L--_

__L___ -10

~

.!- •••••••••••••••} p:.. .. __L~I..L&.&.&...&.&:...;..;;_.__....;.a.:,._J....oIII:::-...:.......-__L______I

-5

0

30

20 10 ~--.....L.-_---L..e.--_......&......-_~ _ _I . _

O'----_---&----o&.--"-

-20

-15

-10

-5

---&-____'

0

Energy (eV)

Figure 9.7

Total and partial densityof state of FeS2 (100) surface

The atomic and bond overlap population analysis of pyrite (100) surface is listed in Table 9.1 and Table 9.2. The overlap population may be used to assess the covalent or ionic nature of a bond. A high value of the bond population indicates a covalent bond, while a low value indicates an ionic interaction (Segall et al., 2002). It can be seen that Fe- Sand S- S bond overlap population on the surface is smaller than those of properties on bulk FeS2. The net charge of Fe atom is varying from-0.12 to 0.05 and that ofS atom is varying from 0.09 to -0.12. It indicates that when a new surface is formed, there exists a process of electron transfer from Fe dangling bond to S dangling bond. This conclusion is in good correspondence with the Nesbitt's DFT works (1998). In this situation, surface Fe and S atom have more ionic properties. Both Fe2+ and S2- have high electrochemistry reduction activity, as is the base for oxygen adsorption. Table 9.1 Atomicpopulation of bulk FeS2 and FeS2(100) surface

Bulk Surface

Atom S Fe SI Fe1

s 1.81 0.31 1.86 0.32

p

4.10 0.59 4.26 0.39

I-atom on the surface; 2-atom below the surface.

226

d 0.00 7.29 0.00 7.22

Total 5.91 8.19 6.12 7.93

Net charge 0.09 -0.19 -0.12 0.07

9.2

Density Functional Theory Research on Oxygen Adsorption on Pyrite (100) Surface Table 9.2

Bond overlap population of FeS2 and FeS2(100) surface Bond S-S Fe-S Sl_S2

Bulk Surface

Population overlap 0.26 0.34 0.26 0.28

Fe 1-S1

1- atomon the surface;2- atombelowthe surface.

9.2.4 Oxygen Adsorption The adsorption energy of oxygen on the FeSz(100) surface is presented in Table 9.3. It can be seen that the adsorption energy of the parallel form 0 is found to be 0.34 eV higher than that of vertical form O. It indicates that the dominant adsorption model may be the parallel form as shown in Fig. 9.1. It is a reasonable assumption model considering that the magnitude of adsorption energy in Table 9.3 is in the same order with that of Oz adsorption on metal oxide surface reported by other authors (Zhang, 1996; Bechtold and Schennach, 1996; Sasaki et al., 2002). The bond length variation before and after optimization is also shown in Table 9.3. Table 9.3

Summary of results for the FeS2(100)/02 adsorption system

Adsorption geometry

Adsorption energy (eV)

0-0 bond length(nm)

0.20 0.34

0.121 0.121 0.135 0.137

Initial configuration Optimized

Vertical form Parallel form

Fe 1-Obond Fe2-Obond length (nm) length (nm) 0.196 0.196 0.199 0.187

0.196 0.197

It follows that in the adsorption state, the 0-0 bond length varies from 0.121 to 0.135 and 0.137 nm for vertical and parallel adsorption forms, respectively. This bond length is close to the 0-0 bond length in peroxides (0.149 nm). It means that after adsorbed by the FeSz surface, the oxygen is more like peroxides rather than Oz. It is clearly in line with the works reported by Ahlberg and Broo (1996a,b,c). The calculated DOS of the parallel Oz adsorption structure is given in Fig. 9.8. It can be seen that the shoulder peaks at -10 eV is the bonding interaction between the 0 (2p) and the Fe (4s), (4p) and (3d) orbital. The antibonding counterpart starts at 0 eV above the Fermi level and is essentially composed of the Fe (3d) orbital and 0 (2p) orbital. The area from -7.5 eV to 0 eV (and above) concerns the interactions of the 0 (2p) with the Fe (4s) and (3d) for the peak. According to the above calculation and discussion, some conclusions can be summarized as follows. 227

Chapter 9 Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

The calculated bulk properties of FeS2 are in agreement with the recent experimental value. The frontier orbital is predominantly occupied by Fe (3d) orbital. It is also an exemplification for the feasibility of using DFT in this system. When the crystal is cleaved from (100) directions, an only the Fe- S bond was broken. Thus, it will produce dangling bonds and surface states. In the formation of the surface, there exists a process of electron transfer from Fe dangling bond to S dangling bond. In this situation, surface Fe and S have more ionic properties than atom properties. Both Fe2+ and S2- have high electrochemistry reduction activity, as is the base for oxygen adsorption. From the viewpoint of adsorption energy, the parallel form 0 adsorption is in preference. The result also shows that after adsorbed by the FeS2 surface, the O2 is more like peroxides.

3.01

2.0 1.0

0.0

0. _~

-20

3.01 >' 2.0 ~

I ~:~ _IL-A

~ a)

r.FJ.

oc

-20

J~eo

~~~~

+

-15

-10

Illl

~=-

........

-5

_~_.........c.._---..1-_----J

I

0

o

~_~~~~....,III!::====-----I---==-____I.___====___..L.___=______=::IIiI ~ ~------~

-15

-10

5

-5

_

___....L_ I

0

____l

5

~f~~~ FeofFe_s~

!

-20

-15

I

!

-10

-5

0

5

3.01

s 2.0 1.0 ~n, 0.0 _ .........-L..~~~"---L-------ooolIl!!..___...L-_~~~~~ ~_ -20 o -15 -10 -5 Energy(eV)

~

____l..._ I _

____J

5

Figure 9.8 Total and partial density of state of02/FeS2 (100) system

9.3 Density Functional Theory Research on Activation of Sphalerite Sphalerite, which is also known as Blende, is an important mineral of zinc. Most natural sphalerite contains iron more or less in lattice with the amount depending on the chemical environment and temperature (Lusk et aI., 1993). High iron sphalerite is called marmatite. The studies of the electronic structure and surface properties of ZnS and transition metal doped ZnS are of interest from both a fundamental and practical perspective. As discussed in Chapter 6, sphalerite has 228

9.3 Density Functional Theory Research on Activation of Sphalerite

a poor flotation response to collectors due to the solubility of Zn-xanthate species, but it can be activated by other transition metal ions such as Cu(II), Pb(II) and Fe(II) (Finkelstein,1997a,b; 1999) resulting in an excellent flotation response . It is generally accepted that Cu2+ ion can replaces the Zn2+ at the mineral surface and the xanthate collector will react with the surface Cu ions. Although it is still not clear about the effect of Cu2+ on surface properties of ZnS, we believe that there exist some relation between the doped ions and surface properties of ZnS. We employ density functional theory to calculate the electron structures of bulk sphalerite, and the pure and doped ZnS (110) surface in order to explain the effect of Cu and Fe ions on the properties of ZnS and the mechanism of activation flotation of sphalerite .

9.3.1 Computational Methods The electric-structure-calculation presented here is performed using the CASTEP computer code, which is based on density functional theory, aided by the CERIUS2 graphical front-end. The wave functions are expended in a plane wave basis set, and the effective potential of ions is described by ultrasoft pseudo potential. The structure of sphalerite is analogous to the diamond structure as shown in Fig. 9.9. The structure isa unit cube with zinc atoms at the corners and face centers. The space group of sphalerite is F-43 m. The initial ZnS lattice constant used in calculation is a = 0.543 nm. The distance between sulfur and its neighbor zinc is 0.234 nm and the distance between two neighbors S or Zn atom is 0.383 nm. The quality of pseudo potential is tested to optimize the structure of bulk ZnS and to compare with the experiment data. In all calculations, the generalized gradient approximation (GGA)-PW91 is used for exchange-correlation functional.

Zn

s

Figure 9.9 Crystal structure of sphalerite and marmatite

229

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

The most important ZnS surface is the (110), which is the most common growth surface and is also the perfect cleavage surface. Therefore, the calculation is based on the ZnS (110) surface. The surfaces are cleaved from the bulk ZnS with the optimum unit cell volume determined using the GGA with CASTEP. The Cu and Fe doped surfaces are built by the substitution ofCu or Fe for Zn atom on the cleaved surface. A vacuum spacing of 1.5 nm is inserted in the z-direction to form a slab and mimic a 2D surface. In order to eliminate the interactions between mirror images in the z-direction due to the periodic boundary conditions, in test calculations, we have done some total energy calculation to find a proper thickness of slab. The result shows that 1.5 nm is a desirable thickness. In the calculation, we have examined the effect of some parameter on total energy and structure of surface and crystal. These parameters include plane wave cutoff energy, k-point and SCF convergence criteria, and the geometry optimization convergence criteria for the geometry optimization task. The calculation results indicate that a plane wave cutoff energy of 280 eV and Monkhorst-Pack k-point sampling density of 4 x 4 x 4 are sufficient for the lattice 5 constant and total energy to converge to within 0.0005 A and 10- eV respectively. For surface relaxation, a plane wave cutoff of 280 eV and a 4 x 4 x 1 k-point mesh are sufficient to converge the surface geometry to within 0.001 A and relaxed 2 surface energies to within 0.001 J/m •

9.3.2 Bulk ZnS Properties The energy bands of the frontier electrons in semiconductors consist of a valence band fully occupied by electrons at low levels, a vacant conduction band at high energy levels; and a forbidden band called the band gap separates these two bands. Figure 9.10 shows the calculated band structure of sphalerite along the selected high-symmetry lines within the first brillouin zone of the FCC lattice. The corresponding total density of state (DOS) and partial DOS of every element are shown in Fig. 9.11. In Fig. 9.10 the band gap is calculated to be 2.7 eV. It is smaller than the experiment band gap, which is measured to be 3.6 eV (David, 2000). The result is in good agreement with other DFT works (Edelbro et aI., 2003; Vanghan et aI., 1997). The S (3s) orbital is located in the interval from -14.5 eV to -12.5 eV in the valence band. This is also in agreement with the works reported by Jellinek et aI., (1974). In bulk ZnS, each Zn atom is coordinated with 4 S atoms and each S atom is coordinated with 4 Zn atoms. Zn and S atoms have two free electrons and six free electrons respectively, which are shared by four Zn- S bonds. These bonds are composed of Zn (Sp 3) hybridization orbital and S (Sp3) hybridization orbital. Figure 9.11 also shows some hybridization of Zn(4s, 4p) and S (3s, 3p) orbital in the interval from 0 to -5 eV, this is the area of Zn-S bond. The bottom of the conduction band has almost equal amounts of Zn(4s) and S (3p) orbital character and is Zn- S antibonding area. Zn (3d) orbitals are 230

9.3 DensityFunctional TheoryResearch on Activation of Sphalerite

located at the narrow interval from -5 eV to -7.5 eY, it is more positive compared with the XPS studyof sphalerite.

6

6

3

3

>' 0 ~ g= -3

0

>' ~-3

00

u (1) a)

~

e.o &3 -6 (1)

_-=========-

or:/'l -6~ o

-9

-9

-12

-12

-15

-15

Q

Q

F

z

o

G

10

kspace

(a)

20

30 40 50 Energy(eV)

60

(b)

Figure 9.10 The calculatedband structure of sphalerite 2

S

>' ~ =0 tl 00

0

u (1) a)

2

-15

r:/'l

0 0

-10

-5

10

Zn

8 6

4

,-15

-10

15

14 12 10

-5

-s

..

0 Energy(eV)

-p ....... d

5

10

15

Figure 9.11 The calculatedpartial densityof state of ZnS

Theatomic andbondoverlap population analysis of bulkZnSis listed in Table 9.4. The overlap population may be used to assess the covalent or ionic nature of a bond. A high value of the bond population indicates a covalent bond, while a low 231

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

value indicates an ionic interaction. Table 9.4 indicates that the Zn- S bond has both covalent and ionic nature. And the value of net charge suggests that in Zn-S covalent bond, S atom shares more electrons. Table 9.4

Atomic and bond overlap population of bulk ZnS

Atom

s

p

d

Total

Net charge

S

1.83

4.69

0.00

6.52

-0.52

Zn

0.86

0.65

9.98

11.48

0.52

Overlap population

Zn-S

0.46

Relaxation and Properties of ZnS (110) Surface

9.3.3

ZnS(110) surface consists of parallel zigzag chains with equal numbers of zinc and sulphur ions (see Fig. 9.12(a» . It is a charge neutral surface. Relaxation of the ZnS (110) surface has been performed using GGA with eASTEP. Some pioneering works show that there is a negligible displacement of ions below the second and third atomic layer. Therefore, in relaxation calculation, only the atoms on the first layer of the surface are allowed to move. The surface structure and ionic displacement vectors for the (110) surface are shown in Fig. 9.l2(b). Ionic displacements due to surface relaxation are presented in Table 9.5.

(a)

(b)

Figure 9.12 The model of ZnS (110) surface and schematic relaxation of surface (a) ZnS (lID) surface; (b) Ionic displacement vectors in surface relaxation Table 9.5 Atom Zn S

232

The ZnS (110) surface ionic displacements due to surface relaxation Coordination

Ax(nm)

~y(nm)

Llz(nm)

3 3

0.001 -0.001

0.001 0.005

-0.02 0.009

9.3

Density Functional Theory Research on Activation of Sphalerite

These results suggest that the most significant relaxation of the ZnS (110) surface is a downward displacement of the surface Zn atoms by approximately 0.02 nm. The surface S atoms relax out the surface by about 0.01 nm. The band structure and partial density of state (PDOS) of relaxed ZnS (110) surface are illustrated in Fig. 9.13. The atomic and bond overlap population analysis of ZnS (110) surface is listed in Table 9.6. It shows that the band gap of ZnS (110) surface is 1.5 eV, and it is smaller than that of bulk ZnS. The reason for band gap

--------..:::::

~

3 ~

o

>'

~

~

(!)

~ -3 ..... Q) ~

u.:l

---- -

-----

:::::::----

-6 -9

-12 G

Q

F

G

Z

kspace

(a)

1.0

l:t

0.5

>' ~ rJ)

~

0

.b

0.0

u Q) Q)

-15

[/)

0

0

c,

. /1 -10

Zn on surface

-5

0

5

10

-5 0 Energy(eV)

5

10

S on surface

2

-15

-10

(b)

Figure 9.13 (a) Calculated band structure of ZnS (110); (b) Calculated PDOS of ZnS (110)

233

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

Table 9.6

Atomic and bond overlap population of ZnS(lIO) surface

Atom SI

s

p

d

Total

Net charge

1.85

4.73

0.00

6.58

- 0.58

S2

1.84

4.70

0.00

6.54

- 0.54

l

0.86

0.65

9.98

11.48

0.52

0.61

0.79

9.98

11.37

0.63

Zn

Zn 2

Overlap population

Znl-S I Zn l - S2

0.40

Zn 2-S2

0.58

0.44

I - atom on the first layer; 2- atom on the second layer.

reduction is due to the rupture of the ZnS bond, which makes the surface Zn and S atom unsaturated and produces surface state. There is an obvious difference between PDOS of surface and that of bulk. A 1.2 eV shift of S (4s) towards the valence band can be observed due to bond-broken. After relaxation, the surface Zn-S bond becomes more ionic because the overlap population of Zn-S bond becomes smaller compared with that of Zn-r-S bond in bulk ZnS. This phenomenon can be attributed to the auto-adjusting behavior to compensate the production of dangling bond. In this regard, surface Zn and S atom have more ionic properties.

9.3.4 Relaxation and Properties of ZnS (110) Surface Doped with Cu 2+ and Fe2+ The models of the ZnS (110) surface doped with Cu2+ and Fe2+ ions are presented in Fig. 9.14. These models are relaxed using GGA with CASTEP. The rule of

(a)

(b)

Figure 9.14 Model of transition metal ions doped ZnS (100) surface (a) Top view; (b) Side view

234

9.3 Density Functional Theory Research on Activation of Sphalerite

relaxation is the same as that of the ZnS (110) surface. The Ionic displacements due to surface relaxation are presented in Table 9.7. It can be seen that the most remarkable displacement is the shift of the Zn atom along z-direction; this is similar to the relaxation of the ZnS (110) surface. When the Zn atom is substituted by Cu or Fe ions, the extent of surface relaxation becomes smaller. Table 9.7 Ionic displacements of the doped ZnS (110) surface due to surface relaxation Surface

Atom

Coordination

Ax(nm)

~y(nm)

~(nm)

Zn

3

0.001

0.001

-0.010

Cudoped

Fe doped

S

3

-0.0011

0.0041

0.0090

Cu

3

0.0047

0.0001

-0.0001

Zn

3

0.0012

0.0011

-0.015

S

3

-0.0009

0.0035

0.0070

Fe

3

0.0032

0.0004

0.0000

The PDOS of the relaxed surface doped with Cu2+ and Fe2+ is presented in Fig. 9.15 and Fig. 9.16 respectively. It can be seen that the valence band of the surface doped with Cu2+ is predominantly composed of Cu (3d) orbital which extends from 0 to -5 eYe This is in excellent agreement with Andrea's XPS studies (1999). There is no obvious gap between the occupied and unoccupied levels. The S (3p) and Zn (4p) orbits are located at the bottom of the conduction band. -s

3

-p

2

>' ~

....... d

S

1 0 -20

8 c: 6 0 ....

-15

-10

~

t)

Q)

1) rF1

2 0 0 0

0..

-20

8 6

-15

0

-10

-s

.... . . . ....

-p

·······d

-5

0

-15

Figure 9.15

10

-p

........-.. : ...... -20

5 -s

eu

4

2 0

10

5

f\t

Zn

4

-5

-10

-5

Energy(eV)

0

·······d

5

10

The PDOS of the Cu2+ doped ZnS (110) surface

235

Chapter 9 Molecular Orbital and Energy Band Theory Approach of Electrochemical ... 16

12

-5

Zn

-p

8

....···d

4

>'

OL....-_.l..--_.1..----:....J....-.-.,;:.....-I!l......3ooo.......

-15

-10

-5

""'--..-:::....::=---.~~ _ _....L...________l

o

10

5

~

§

2

Q)

1

8

OL---o..,L...-~_"---~~---..o/:.....L--_~~...J...........~~........::=:...~"'____L..______J

~OJ

-5

-p

·······d

(/'J

~

-15

-10

o

-5

5

10

9

Fe

6

-5

-p

....... d

3

o

. -15

-10

-5

0

5

10

Energy (eV)

Figure 9.16 The PDOS of the Fe2+ doped ZnS (110) surface

SimilarPDOS distribution can be seen on the ZnS surface doped with Fe ions. The dominant state in valence band is Fe (3d) orbital, and the conduction band is composed of S (3p) and Zn (4p) orbital. This result indicates that doping Cu or Fe ions on the ZnS surface reduces the band gap of the ZnS. This kind of reductionwill produce lot of surface state in bulk ZnS forbidden band. It is reported that the band structure of ZnS doped with transitionmetal ions is remarkably differentfrom that of pure ZnS crystal. Due to the effect of the doped ions, the quantum yield for the photoluminescence of samples can be increased. The fact is that because more and more electron-holes are excited and irradiative recombination is enhanced. Our calculation is in good correspondence with this explanation. When the ZnS (110) surface is doped with metal ions, these ions will produce surface state to occupy the valence band and the conduction band. These surface states.can also accept or donate electrons from bulk ZnS. Thus, it will lead to the improvements of the photoluminescence property and surface reactivityof ZnS. In flotation, when sphalerite is activatedby Cu2+ or Fe2+, the ZnS surface will exhibit good reactivity to organic collector. Our calculation shows that when the surface is doped by transition metal ions, the surface ionswillbe rendered more ionic property, whichbenefits the interaction between the mineral surface and the collector anions. It gives more profound explanation for Cu2+ activatedbehavior to ZnS. Because the oxidationof sulphide has a pronounced effect on sulphide mineral flotation, oxidationwill produce metal ions on the mineral surface and these ions will react with collectors to render the surface hydrophobicity. From the DOS shown in Fig. 9.13, Fig. 9.15 and Fig. 9.16, marmatite and (Zn, Cu) S are the intrinsic semiconductors and.sphalerite is a broad band semiconductor. The top of the valence band of above three materials are dominantly occupiedby Fe (3d), 236

9.3

Density Functional Theory Research on Activation of Sphalerite

S (3p), and Cu (3d) orbit, respectively. According to the frontier orbital theory, the electrons in the highest occupied state are most easily bound and have an unexpectedly great significance for the chemical reactivity of materials. It indicates that the different reduction or oxidation would happen on the three mineral surfaces in the pulp during flotation system. Based on Pattrick et a1.(1999), mannatite is easily to be oxidized than sphalerite and Fe at the (Zn, Fe) S surface is predominantly bonded to O. There is also some literatures to show that the ZnS surface is resistant to oxidation but oxidation of the ZnS surface after Cu activation is apparent. All these studies imply that ZnS has low electrochemistry activity. This is because of the following reasons: ZnS is a broad band semiconductor, the electron in the fully occupied valence band is difficult to be excited up to the conduction band. There is less free electron in ZnS, and then it is not easy to accelerate the electrochemistry oxidation of surface. That is to say, the dissolution of ZnS from mineral surface is difficult and has little chance to react with thio-collectors. When substituted by Cu or Fe ions, the forbidden gap of doped ZnS will decrease and the quantity of free electrons in ZnS will increase. This variation will enhance the electrochemistry activity of ZnS.

9.3.5

Effects of Doped Ions on Mixed Potential

Marmatite has a narrower band gap than (Zn, Cu)S, thus it should be more easily oxidized than (Zn, Cu)S. But in fact, the sphalerite after Cu activation has the most excellent flotation response using xanthate. These phenomena can be explained by the mixed potential theory. According to the mixed potential theory, an anodic reaction can occur only if there is a cathodic reaction proceeding at finite rate at that potential (Rand and Woods, 1984). For the flotation systems, the cathodic reaction is usually given by the reduction of oxygen. The corresponding anodic reaction involves interaction of xanthate on the sulphide minerals in various ways, including the reaction of xanthate with the sulphide mineral (MS) to form metal xanthate and the oxidation of xanthate to dixanthogen (X 2) at the mineral surface. The mixed potential of the sulphide mineral in the flotation pulp will determine the oxidation product on its surface. If the mixed potential of the mineral in the presence of oxygen, xanthate and other reagents is above the mixed potential for the X-/X2 redox couple, then the reaction will produce dixanthogen on the surface. If the mixed potential is lower than the X"/X 2 redox couple, metal xanthate reaction will take place rendering the surface hydrophobic. A lot of work show that the existence of Fe ions on solid surface can catalyze the reduction of oxygen. It indicates that the rate of oxygen reduction on marmatite is larger than rate of reduction on (Zn, Cu)S, this will cause the mixed potential of marmatite to be higher than that of (Zn, Cu)S and ZnS. This process can be illustrated in Fig. 9.17. 237

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

- - - - Mixed potential of marmatite - - - - - Mixed potential of(Zn, Cu)S

Figure 9.17

Schematic of mixed potential marmatite and (Zn, Cu) S

9.4 The Molecular Orbital and Energy Band Discussion of Electrochemical Flotation Mechanism of Sulphide Minerals As the discussions in the previous chapters, some sulphide minerals possess stronger collectorless floatability and collector floatability at certain potential range. Normally, according to the mixed potential mode, an anodic reaction can occur only if there is a cathodic reaction occurring at that potential and at the same time. For the flotation system, the cathodic reaction is usually given by the reduction of hydration oxygen. The corresponding anodic reactions involve that in different ways. The self-oxidation of sulphide mineral results in the formation of elemental sulphur showing self-induced collectorless floatability or produced sulfur-oxy and hydroxyl species showing no flotation. The reaction of the collector such as xanthate on sulphide mineral may form metal salt or dixanthogen to exhibit collector flotation behavior. In the following section, we will discuss the anode oxidation and cathode reduction occurring at the sulphide/solution interface from molecular orbital and energy band theory.

9.4.1

Frontier Orbital of Collector and Oxygen

Dimethyl-dithio-amine-carbonate (DDTC) and butyl xanthate (BX) are often used in the flotation of sulphide minerals. Their structures are displayed in Fig. 9.18. Shown in the same figure. is the structure of hydration oxygen. We take ab initio theoretical calculations on the orbital energy of BX and DDTC and the hydration oxygen atom. All the calculations are taken by Gaussian98 software. The geometries of the compounds and the frequencies are evaluated using the DFT level of the three parameter compound functional of Becke (B3LYP). The 6-31G(d) basis set is used for all atoms. The geometry structures of neutral molecules and ion molecules are optimized under no constraint. The resulting HOMO and LUMO energy and orbital symmetry of BX and DDTC are listed in Table 9.8 and Fig. 9.19. 238

9.4 The Molecular Orbital and Energy BandDiscussion ofElectrochernical ...

s

(a)

»:

/e

/

(b)

<,

<, <,

(e)

Figure9.18 The structures of butyl xanthate, dithiocarbamate and hydration oxygen molecules (a) Butyl xanthate; (b) Dithiocarbamate; (c) Hydration oxygen

(b)

(a)

(e )

Figure9.19 The highest occupied orbit of BX, DDTC and lowest unoccupied orbitof hydration oxygenatom (a)Butylxanthate; (b) Dithiocarbamate; (c) Hydration oxygen

239

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

Table 9.8

Orbital energy ofBX, DDTC and hydration oxygen atom

Orbital energy (eV)

Xanthate

DDTC

Hydration 0

OH-

X2

HOMO

-3.4 3.6

-3.6 3.8

-13.9 -3.9

-7.23 -1.23

-9.23 -2.23

LUMO

For simple organic molecules, the MOs are occupied by two electrons (spin up and down) up to the highest occupied MO (HOMO). The HOMO energy level is on behalf of the ability of the molecule donating electrons. The higher the HOMO is, the easier it donates electrons. All MOs above the HOMO are unoccupied, starting with the lowest unoccupied MO (LUMO). LUMO represents the ability of the molecule accepting electrons. The lower the LUMO is, the easier it accepts electrons. It can be seen from Fig. 9.19 that the HOMO orbitals of BX and DDTC mainly spread over three atoms of the C- S- S group. It indicates that the functionalization group of the C - S- S strongly affects the reduction potential and oxidation potential of molecule. Table 9.8 shows that the energy level of xanthate HOMO is -3.4 eY, which is higher than that ofDDTC, -3.67 eYe It suggests that xanthate can donate electrons to an electrode more easily than DDTC. As for hydration oxygen atom, the energy levels of LUMO and HOMO are, respectively, -3.94 eV and -13.9 eVe

9.4.2 The Molecular Orbit and Energy Band Discussion of Collectorless Flotation of Galena and Pyrite The structures of the energy bands of galena and pyrite are shown in Fig. 9.20. From Section 9.2.2, we know that the Fe (3d) non-bonding orbitals are located at the top of the valence band and extend to just below the Fermi level, and that the part of Fe (3d) orbital and antibonding Sp3 orbitals of sulfur contribute to the conduction band. Similar calculations can be applied to galena. The valence band of galena is mainly made up of S (3p) orbit. Pb (6s) band and (6p) comprise the conduction band of PbS. It should be pointed out here that we obtained all the energy levels of band using a Fermi level as 0 reference in the calculations in Section 9.2.2. The energy level is only a relative value. If we discuss the electron transfer between mineral electrode and pulp electrolyte, we should know the energy level of band using vacuum level as 0 reference. According to the research of Chemyshova (2003), the Fermi levels of pyrite and galena are, respectively, -4.4 eV and -3.86 eV in vacuum. The energy levels of galena and pyrite in Fig. 9.20 are the value using vacuum level as 0 reference. Like the HOMO in a molecule, the Fermi level in sulphide mineral is on behalf the ability to donate or accept electrons, The more negative the Fermi level, the more up-shifted the electro potential, and so the greater the tendency to accept electrons from the adsorbate to the electrode. 240

9.4 TheMolecular Orbital and EnergyBandDiscussion of Electrochemical ... .------ 0 eV, vacuum

OeV, vacuum

Energy ~onduction band

Energy ~onduction band

> Q)

Fermi level ~ - 3.86 eV ~ - - - - - - - ':!

I

- 3.94 eV

III~'!>.....-_ O(2H20) LUMO

13.94eV -------

-

> Q)

Fermi level

~--O(2H20)

d

LUMO

-4.4 eV -------

OHHOMO

Figure 9.20

Illustration of electron transfer between sulphide surface and

hydrationoxygen atom showingthe mechanism of collectorless flotation of galena and pyrite

It can be seen from Fig. 9.20 that the top valence band of galena is -3.86 eY, which is higher than that of the LUMO of hydration O. It is feasible in thermodynamics to transfer electrons from galena valence band to LUMO of hydration O. The valence band of galena is mainly made up of S (3p) orbit. Galena is oxidized to form elemental sulphur, which shows surface hydrophobic and good collectorless floatability. For pyrite, it is easy to see that the LUMO of hydration 0 atom is lower than the bottom of pyrite conduction band (-3.5 eV) but higher than the top valence band (-4.4 eV). Therefore, the electron transfer process occurring at pyrite surface is different from the process at galena surface. The conduction band of pyrite consists of the part of Fe (3d) orbital and antibonding Sp3 orbitals of sulfur. The oxidation of pyrite easily produces iron hydroxyl species or sulphur-oxy species to exhibit surface hydrophilic and poor self-induced collectorless floatability.

9.4.3

The Molecular Orbit and Energy Band Discussion of Collector Flotation of Galena and Pyrite

Figure 9.21 shows the conduction band and valence band within pyrite along with the orbital energies (HOMO and LUMO) of BX and DDTC in solution. On the left side, the bottom of the conduction band (unoccupied band) of pyrite has a 241

Chapter 9

Molecular Orbital and Energy Band Theory Approach of Electrochemical ...

lower value (-3.5 eV), which is lower than the HOMO of butyl xanthate (-3.4 eV). It is, therefore, thermodynamically favorable for an electron to jump from the xanthate molecule to the pyrite electrode. Xanthate molecule loses one electron to become dixanthogen, which renders pyrite hydrophobic. However on the right side, the bottom of the conduction band of pyrite is above the HOMO of DDTC (-3.61 eV). It is thermodynamically unfavorable for the electron transfer to occur. It suggests that the formation of disulphide of DDTC on the pyrite surface is more difficult than xanthate. It requires stronger oxidation atmosphere. oeY, vacuum

.------ 0 eV,vacuum

Energy ~ Conduction band

Energy

HOM0 BX

==ii~""~~~3.4 Easy

Fermi level

> o

LUMO DDTC 2

======lr~ Conduction band

eV

HOMO DDTC

--3.6eV Difficult

>

(l)

(l)

Fermi level

0\

-4.4

0\

o

-1-1-1-1-11--

eV - - -

--

~

~ Valence band ~

Pyrite

I DOTe

Figure 9.21 Illustration of electron transfer between pyrite surface and collector showing the mechanism of collector flotation of pyrite

Figure 9.22 shows the band energy level of galena and frontier MO ofBX and DDTC ion. It can be seen that the highest occupied orbital of BX and DDTC molecule is higher than the bottom conduction band of galena (-3.28 eV). It is difficult to transfer electrons from molecule to mineral surface, i.e., the disulphide of collector such as xanthate and dithiocarbamate may not be easily formed on the galena surface. However, on the other hand, the top valence band of galena is -3.86 eY, which is higher than that of the LUMO of hydration oxygen. The oxidation of the galena surface produces lead ion and sulphur or sulphur-oxy species. The collector metal salts can be formed on the galena surface through the reactions between lead ion and collector. Takahashi (1991) also reported that in the adsorption of EX- on the ionic solids electron transferred from the solids to EX-, while they transfer from EX- to the solids in the adsorption of EX- on the covalent solids. 242

9.4 TheMolecular Orbital andEnergy BandDiscussion of Electrochemical ...

o eV, vacuum

....---- 0 eV, vacuum

LUMO DDTC 2 Energy

Energy ~ Conduction band

~ Conduction band

HOMOBX

Fermi level

> ~

HOMO DDTC

-3.4 eV Difficult

V")

~>

Fermi level

o

-3.86 eV

---1-1-1-1-1-11-_

~

Difficult

o

IBI-

-3.86 eV ---------

~ Valence band ~

~ Valence band

Galena

----3.6eV

Q)

BX

Galena

1 ODTC

Figure 9.22 Illustration of electron transfer between galenasurface and collector showing the mechanism of collector flotation of galena

243

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals

Abstract This chapter is mainly about potential controlled flotation. First, the factors affecting potential controlled flotation are discussed, such as potential modifiers, pH modifier, frother, conditioning time, surface pretreatment and grinding environment. It is found that the flotation behavior of minerals modified by different potential modifiers or pH modifiers may differ from each other even at the same potential or the same pH. And other factors are also investigated to find the way of influencing the pulp potential to control the flotation behavior of minerals. Thereafter, some examples about potential controlled flotation are presented which are the flotation of copper sulphide minerals and the flotation of lead-zinc-iron sulphide minerals. As the last part, origin potential control flotation (OPCF) is introduced. The factors coupled with OPCF including grinding, lime dosage, aeratation and reagent schemes are all dicussed, which are different from the conventional flotation. In the end listed are some applications of OPCF technology. Keywords potential controlled flotation; potential modifier; original potential; original potential control flotation (OPCF)

10.1 Technological Factors Affecting Potential Controlled Flotation Separation of Sulphide Ores The behaviors and mechanisms of electrochemical flotation of sulphide minerals have been extensively investigated in laboratories. In order to improve the selectivity of separation of a mixture of sulphide minerals and to optimize sulphide ore flotation by electrochemistry control, it is required to understand the effects of technological parameters such as potential modifier, pH modifier, frother, galvanic interaction between sulphide minerals, conditioning, pretreatment and grinding media etc. (Trahar, 1984; Nakazawa and Iwasaki, 1986; Yelloji and Natarajan, 1990; Wang et al., 1991a,b,c,d; Wang et al., 1992).

10.1.1 Potential Modifiers Potential modifiers are one of the most important factors affecting the

10.1

Technological FactorsAffecting Potential Controlled Flotation Separation ...

electrochemical controlled flotation of sulphide minerals by the addition of which a genuine redox potential is established which has shown its significance in the flotation of sulphide minerals. It should be noted here that the influence of potential modifiers on the electrochemical controlled flotation of sulphide minerals is different for various modifiers even if under the same potential. Figure 10.1 reflects the flotation behaviors of chalcopyrite in the presence and absence of butyl xanthate as a function of potential. It is evident that the flotation behavior of chalcopyrite is different at the same potential modified by ammonium persulphate and hydrogen peroxide. The initial potential of collectorless and butyl xanthate flotation of chalcopyrite are around 0 V. However, the upper limit potential of collectorless and butyl xanthate flotation of chalcopyrite is different using different potential modifiers. Chalcopyrite appears almost complete flotation at 0.3 V extended to 0.6 V when the potential is modified by ammonium persulphate, but it is completely depressed above 0.3 V when the potential is modified by hydrogen peroxide. Perhaps hydrogen peroxide is a stronger oxidizing agent than ammonium persulphate and eliminates the over potential of oxidation of sulphur to oxy-sulphur species which confers the chalcopyrite surface hydrophilic easier in the presence of hydrogen peroxide than in ammonium persulphate. Reyes and Trahar (1977) reported that the recovery of chalcopyrite is much different at the same potential modified respectively by potential raising agent. 100 r - - - - - - - - - - - - - - - - - - , 80 pH=10 : : : (NH 4hS20 S

-+~

20

Na2S204

::.: H202

0L..--_---a...._ _....L--_----"-_ _- - ' - - _ - - - - ' -0.2 0 0.2 0.4 0.6 0.8

Potential(V,SHE)

Figure 10.1 Recovery of collectorless (dashed line) and butyl xanthate induced flotation(solid line) of chalcopyrite as a function of pulp potential. Solid line-butyl xanthate: 2.9 x 10-5 mol/L, dashed line-collectorless flotation: Butyl ether alcohol 7.5 mg/L (Wang, 1992)

Figure 10.2 demonstrates the flotation recovery of pyrite vs. potential with butyl xanthate as a collector. It is follows that pyrite exhibits good floatability over a wider potential range extending from -0.1 V to 0.3 V when the potential is modified by sodium sulphide and ammonium persulphate. However, the potential range of pyrite flotation is diminished from 0.2 V to 0.3 V when the potential is modified by sodium dithionite and ammonium persulphate. It suggests that the reduction environment induced by reducing agents may be different for the 245

Chapter 10 Electrochemical FlotationSeparation of Sulphide Minerals

collector action even if the potential value is the same. Pyrite exhibits better floatability by using sodium sulphide as a potential modifier than by using sodium dithionite because the former exhibits in fact sulphur-induced flotation behavior in addition to collector action. 100 • Na2S • (NH4hS20g 6 Na2S204

80 ,-..,

?fe.

'6

60

u

40

Q)

;> 0 Q)

0:::

20

oL . -_ _- 1 -_ _...l:!Jt-_--=:::.....-l..-_ _ -0.4

-0.2

0 0.2 Potential rv SHE)

--J

0.4

.Figure 10.2 Recovery of flotation of pyrite as a function of pulp potential (Butyl xanthate: 2.9 x 10- 5 mol/L; Butyl ether alcohol: 7.5 mg/L;from Wang, 1992)

10.1.2 pH Modifier pH modifier also plays an important role in electrochemical controlled flotation separation of sulphide minerals. Figure 10.3 shows that the upper limiting pH for the depression of collectorless flotation of chalcopyrite is decreased when different pH modifiers are used in the order of sodium hydroxide, lime, sodium sulphide. It can also be seen from Fig. 10.3 that sodium sulphide decreases the pulp potential evidently besides for pH modification. When pulp potential is decreased to reducing environment at high pH by Na2S, the collectorless flotation

----,'"

0.8 ,-.., 0.6

:c~

~-~

~ 0.4

--~

pH modifier

C 0.2

~

0..

• HCI,NaOH o CaO • Na2S

0 -0.2

\

2

4

6

100 --<0,

\

\

"~

60

~

C Q) 40 o~

\.

Q)

~

8 pH

80

~

""

10

20 12

14

0

Figure 10.3 Recovery of collectorless flotation of chalcopyrite and pulp potential as a function of pH (Wanget al., 1991a,b,c,d) 246

10.1 Technological Factors Affecting Potential Controlled Flotation Separation ...

of chalcopyrite is suppressed. The decrease of pulp potential with the increase of pH is related to the kinds of pH modifiers in the descending order of sodium hydroxide, lime and sodium sulphide which is in agreement with the order of upper limiting pH for depression. Figure 10.4 presents the flotation recovery of galena, sphalerite and pyrite vs. butyl xanthate concentration with different pH.modifiers at pH= 12. At the same pH condition, lime shows stronger depressing on these three minerals than sodium hydroxide with butyl xanthate as a collector. Pyrite can be completely depressed at pH = 12 modified by NaOH or lime. Sphalerite can be effectively depressed only by using lime modifying pH to 12. At pH = 12 modified by NaOH or lime, galena remains with good flotation response. Therefore, lime may be the effective pH modifier and depressant for the flotation separation of lead-zinc-iron sulphide ores. Fuerstenau et al. (1968) reported that complete depression of ethyl xanthate flotation of pyrite may occur whether KOH, NaOH, K2C03 or CaO are used for pH control, but the upper limiting pH is variable with different pH modifier. 100 90

,-.....

'$.

-.-:-=:

PbS+NaOH

80 70

•

~

~ 60

-.-.A

..

PbS+CaO

Q)

> 0

50

~

40

u Q)

30

zns~

~~

~ ~NaOH~

o

~

FeS2+CaO

' - - - . . . L - - _ - - L - _ - - - - L . . . . . _ - - - - - - L_ _. L . . - - _ . . . . L . . - _ - - - l

0.0004 0.0008 0.0012 Concentration of collectors (mol/L)

Figure 10.4 Flotation recovery of galena, sphalerite and pyrite as a function of butyl xanthate concentration with different pH modifiers at pH = 12

At the same pH made by different pH modifiers, the different flotation response of one sulphide mineral may arise from its effect on the potential of this mineral electrode. The change of potential of mineral electrodes with time at pH = 12 modified by NaOH and Ca(OH)2 is measured and demonstrated in Fig. 10.5. It follows that a mineral electrode potential increases faster with the time at pH = 12 modified by sodium hydroxide and changes a little at the same pH modified by calcium hydroxide. When pH is adjusted by NaOH, the electrode potential of galena, sphalerite and pyrite increase rapidly, respectively, from -30, -12, and 70 mV at the initial stage to -10, 10, and 110 mV after 50 min. When pH is modified by lime, the electrode potential of galena, sphalerite and pyrite 247

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals

slightly increase respectively, from -40, -12, 60 mV at the initial stage to -36, -8, and 75 mV after 50 min. It is the different influence of pH modifiers on the potential that they show different influence on the flotation of sulphide minerals. 300.---------,

300.-----------.

298 296 294

~ 292

VJ

;;;

290

E

VJ

;;;

-40

~ ......... . .

-60

"----L-----L.----L.-..J...(J-L--....L...-..L..----.L..---J

~ -20

o

---NaOH --6- Ca(OHh

GJ 295 :c

....... NaOH --.-Ca(OHh

~

100

50 500 3000 3500 4000 t(s)

o 200400600

L...-L-..JL......L-.J~-J........IJ-I---'----'--~

(a) Galena

3200 t(s) (b) Pyrite

3600

300.-----------,

295 290

285 (i3'280 ~ 275

....... NaOH ---Ca(OHh

;;; 10

S k:l

5

0

-5

-10 -15 '---"'""'--~____L.."J_I__....L.-"'""'--~ o 500 3000 3500 4000 t(s) (c) Sphalerite

Figure 10.5 The change of potential of mineral electrode with time at pH = 12 modified by NaOH and Ca(OH)2 (KN03 : 0.1 mol/L; BX: 10- 4 mol/L)

10.1.3 Frother The type and addition of frother are found to have a pronounced effect on the collectorless floatability of chalcopyrite (Heyes and Trahar, 1977). The recovery of collectorless flotation of chalcopyrite is much higher using PPG40 than amyl alcohol. The effects of several frothers on the collectorless flotation of some minerals have been tested and the results are presented in Table 10.1. It further provides the evidence that the type of frother produces a markable influence on collectorless flotation of sulphide minerals. The frothers with lower surface tension are more effective in enhancing the recovery of collectorless flotation of sulphide minerals. 248

10.1 Technological Factors Affecting Potential Controlled Flotation Separation ... Table 10.1 Influence of frother on the collectorless flotation of the sulphide minerals (Wang et aI., 1991a,b,c,d) Butyl ether alcohol

Yarmour

MIBC

Mixed alcohol

Surface tension (10- 3 N/m)

61.78

67.43

68.30

68.74

68.74

Separation efficiency of chalcopyrite/pyrite

84.85

63.66

63.65

56.06

52.59

Stibnite

94.0

87.0

85.0

61.0

74

32.3

Arsenopyrite

96.0

74

32.0

Frother (100 mg/L)

Recovery(% )

94.0

Pine oil Alcohol 70.91

10.1.4 Conditioning Time Because the electrochemical controlled flotation of sulphide mineral is possible under adequate oxidizing environment, longer time for conditioning may be useful for the flotation of some sulphide minerals that require oxidation atmosphere. Since iron balls are usually being used as the grinding media in mineral processing operations, the environment created in an iron mill is highly reducing. Figure 10.6 demonstrates the pulp potential as a function of flotation time. It can be seen that if the chalcopyrite ore is ground in a iron mill which set the pulp potential at -0.3 V and the flotation is conducted immediately after grinding, the initial pulp potential is in strong reducing environment -0.3 V and increases slowly to -0.2 V after flotation for 2 min, and continues to increase to +0.2 V after flotation for 8 min. The maximum recovery of chalcopyrite under these conditions can reach only to 61.32%. If the chalcopyrite ore is ground in a iron mill but the pulp is strongly agitated in a tank or in flotation cell for 10 min before flotation, it will 0.6.-------------------. 0.5 ~

~ ~ ~

.~

2o

0.4 0.3 0.2 0.1

0

~ -0.1 ~ -0.2

2

Recovery(%) -+- 61.32

-<>-78.83 76.9

--4r-

-0.3 -0.4 L...-----L._---I..-_.......J...-_..L------'_-----'-~___' o 2 4 6 8 10 12 14 Flotation time(min)

Figure 10.6 Pulp potential as a function of flotation time 1-strong agitating 10 min under air; 2- aeration conditioned 10 min; 3- flotation immediately after grinding

249

Chapter 10 Electrochemical FlotationSeparation of Sulphide Minerals

set the initial potential in oxidizing environment at 0.36 V or 0.3 V respectively, the pulp potential is soon increased to 0.48 V and 0.45 V after flotation for 2 min. The maximum recovery of chalcopyritecan reach to 78.83% and 76.9% respectively. It suggests that the potential controlled flotation of sulphide ores is affected by agitating condition due to its influence on redox atmosphere.

10.1.5 Surface Pretreatment The oxidized or aged sulphide minerals usually appear to have poor floatability which may be improved by surface pretreatment. The influence of surface pretreatment on the collectorless flotation of chalcopyrite and galena is given in Fig. 10.7. It follows that oxidized galena and aged chalcopyrite exhibit poor flotation, which is greatly improved by pretreating with ultrasonic wave, the action of which is to remove the hydrophilic oxidized film on the oxidized mineral surface. Wang and Forssberg (1989) reported that no flotation was observed for oxidized galena, pyrite and arsenopyrite in the absence of EDTA, but flotation was recovered by the addition of EDTA, which was attributed to the removal of the hydrophilic metal hydroxide layers by EDTA from these sulphide surfaces and thereby exposing a sulphur-rich surface. 100 .------------,.-~--------, ~..:::-~ ~~-=i - - . 4

'~

"..-

.-....

'$.

f

Q)

80

\

,,-----.......

,, ,

• Galena • Chalcopyrite

\

'2

-\

60

> o

~ 40 20

o

\

2

468 pH

10

~

12

14

Figure 10.7 Influence of surface oxidation andpretreatment on self-induced flotation of galena and chalcopyrite. Galena: I-untreated; 2- pretreatedby ultrasonic wave. chalcopyrite: 3-aged two month; 4-pretreated by ultrasonic wave (Wang et aI., 1991a,b,c,d)

10.1.6 Grinding Environment When a complex sulphide ore is subjected to wet grinding, electrochemical interactions between the grinding media and various sulphide minerals and chemical or electrochemical interactions involving dissolved species and mineral 250

10.1 Technological Factors Affecting Potential Controlled Flotation Separation ...

surface could be expected. Subsequent flotation of such sulphide minerals would be influenced by the type of grinding media used and dissolved metal ions (Nakazawa and Iwasaki, 1986;Yelloji Rao and Natarajan, 1989a,b,c, 1990). Different grinding media will result in different redox environments of pulp and hence affect potential controlled flotation behavior of sulphide minerals. Heyes and Ralston (1988) reported that chalcopyrite ground in a reducing environment provided by 5 x 10- 3 mol/L sodium dithionite solutions at pH = 9.2 exhibited weak collectorless floatability and was made strongly floatable by increasing the potential above 0 - 0.1 V. Galena in a reducing environment provided by 5 x 10-3 mol/L sodium dithionite solutions at pH = 9.2 is not floatable but can be readily floated by increasing the E; to 0.45 V. In practice, sulphide minerals are frequently ground in an iron mill under strongly reducing conditions owing to the operation of one of the following reactions: Fe 2+ + 2e- =

Fe

(10-1)

EO = -0.44(V) (10-2)

EO

=-0.047(V) (10-3)

EO

=-0.084(V)

which are comparable to the dithionite environment. They show that chalcopyrite and galena are not floatable when ground in an iron mill in which the potential is about -0.25 V but are made strongly floatable by increasing the potential to above 0.1 V for chalcopyrite and 0.2 V for galena. Generally speaking, chalcopyrite and galena are less floatable when ground in an iron mill than in a ceramic mill. Grinding media also have an influence on the collector flotation of sulphide minerals. Grano et al. (1990) found that the floatability of butyl xanthate of chalcopyrite and iron sulphides is lower when the copper ore was ground in an iron mill than in a ceramic mill. They concluded that the introduction of highly electrochemically active grinding media in cast iron would enhance the generation of ferric hydroxides as a result ofthe anodic dissolution of the media which may completely coat the sulphide mineral surfaces leading to depression for the mineral. It would be expected, therefore, that the removal of some of these hydroxides by grinding in an inert environment would allow greater flotation recoveries to be achieved. Cases et al. (1990) reported that only dixanthogen was observed on pyrite after using stainless steel rods, while ferric iron xanthate and physically absorbed xanthate ions were found to be formed when iron rods were used. 251

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals

In lead-zinc-iron sulphide ores, iron ball is a typical grinding medium. Besides, anyone of the three minerals may be regarded as the grinding medium of the other two minerals. The change of potential of mineral electrode with time in different grinding media at pH = 12 modified by Ca(OH)2 was measured and given in Fig. 10.8. It is obvious that by grinding in the iron ball medium the electrode potential of galena, sphalerite and pyrite tends to decrease with the time, respectively descending from -35, 10, and 130 mV at the initial stage to -60, -10, and -30 mV after 7 min. The electrode potential of galena and sphalerite increases slowly with the time when grinding in pyrite medium. The electrode potential of sphalerite and pyrite decreases slowly with the time when grinding in galena medium. Whereas when grinding in sphalerite medium, the electrode potential of galena tends to increase and that of pyrite tends to decrease. Therefore, even if in iron ball grinding medium, different mineral compositions may have variable potential environment accounting for one reason of difficulty in flotation separation of poly-metal sulphideores. 300

300~----------,

---- FeS2 -o-Fe --6- lnS

---- PbS lnS --6- Fe

-0-

~ 210

rJ:1

>E 140 ~

70

o

-50 100 150 200 250 300 350 400 450 t(s) (a) FeS2

100 150 200 250 300 350 400 450 t(s) (b) PbS

300 280

----Fe -o-PbS --6- FeS2

GJ 20

:I: rJ:1

>"'

E

~

- 20 L...-..J.-....&...-L---'---L...-..J.-....&...-L---'---L.--L-~--L-....I 100 150 200 250 300 350 400 450 t(s) (c) lnS

Figure 10.8 The change of potential of mineral electrode with time at different grinding media (KN0 3 : 0.1 mollL; BX: 10- 4 mol/L; pH = 12, by lime)

252

10.2 FlotationSeparation of Sulphide Mineralsand Ores

10.2 Flotation Separation of Sulphide Minerals and Ores 10.2.1 Copper Sulphide Minerals and Ores 1. Chalcopyrite /Galena Flotation separation of chalcopyrite from galena, especially in the copper-lead mixed concentration, is very difficult in plant practice. The potential controlled flotation provides a possible way to separate them effectively. Figure 10.9 presents a flowsheet of collectorless flotation separation of chalcopyrite and galena. Both chalcopyrite and galena are floated in an adequate oxidizing environment at natural pH. The separation of mixed concentrate is conducted at higher oxidizing potential, which is usually modified by H202 or other strong oxidizing agents. Table 10.2 shows the results of flotation separation for the mixture of chalcopyritegalena and quartz according to the flowsheet in Fig. 10.9. The obtained copper concentration assayed 25.78% and recovery is 81%, and the obtained lead concentration assayed 65.7% and recovery is 90% after separation. Mixture(15%PbS, 15%(Cu, Fe)S, 70%Si02) Grinding pH=6.7-7.0 Eh modifying Bulk collectorless flotation pH=9.2-9.5 Eh modifying Separation flotation

Cu cone.

Tailing

Pb cone.

Figure 10.9 Flowsheet of collectorless flotation separation of a mixture of chalcopyriteand galena Table 10.2 Separation resultsof collectorless flotation of a mixtureof chalcopyrite, galena and quartz (Feng et al., 1991)

Ph

Cu

Bulk flotation

Separation

Products

Grade (%)

Recovery (%)

Grade (%)

Recovery (%)

e, (mY)

pH

e, (mY)

pH

Ceramic mill

25.78

81.00

65.70

90.00

300

6.7

480

9.2

Iron mill

20.28

80.88

56.70

73.48

265

7.0

410

9.5

253

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals

Tests have been done further on the separation of a Cu-Pb mixed concentration of ethyl xanthate flotation of copper-lead-iron sulphide ore by E; control modifying with H202. Test results are presented in Table 10.3. It indicates the possibility of selective flotation separations of copper-lead flotation concentration by E; control. The feed of copper-lead mixed concentrated assayed Cu 6.53% and Pb 62.38%. Using hydrogen peroxide as a potential modifier, a copper concentration with 24.19% Cu and recovery with 89% can be obtained after separation. Table 10.3 E h control separation of a mixed concentration of collector flotation of chalcopyrite-galena ore (Wang, 1992a,b) Test 2# (H20 2 : 408 g/t)

Test 1#(H20 2 : 476 g/t) Products

Grade (%)

Recovery (%)

Grade (%)

Recovery (%)

Cu

Pb

Cu

Pb

Cu

Pb

Cu

Pb

Cu cone.

24.19

16.88

89.00

6.43

23.83

18.19

85.98

7.84

Pb conc.

0.95

76.84

11.00

93.57

1.36

74.98

14.02

92.66

Feed

6.53

62.38

100

100

7.2

60.23

100

100

2. Chalcopyrite / Pyrite Potential controlled flotation separation of chalcopyrite-pyrite ores has been extensively tested to be one of the most probable to obtain industrial application on the basis of the fact that chalcopyrite has strong collectorless and collector floatability whereas pyrite has poor collectorless floatability. Figure 10.10 presents the schematic rougher flowsheet of potential controlled flotation separation of chalcopyrite from copper-sulphur ores. Table 10.4 lists the results of flotation chalcopyrite from many types of copper-sulphur ores. It follows that almost similar results are obtained for collectorless and xanthate flotation chalcopyrite from copper-sulphur ores. When the grade of feed ore is about 0.5% - 1.5% Cu, the grade of the obtained rougher concentration can reach to 7% - 17% Cu and recovery is above 90%. When the grade of feed ore is about 3%- 5% Cu, the grade of the obtained rougher concentration can reach to above 17% Cu and recovery is above 95%.

-r

-r Feedore

Feedore

Eh controlled collectorless flotation

~

Cu cone. (a)

Figure 10.10 sulphur ores 254

l

Tailing

~

Eh controlled collectorflotation

Tailing

Cu cone. (b)

Principal flowsheets of potential controlled flotation of copper-

10.2 Flotation Separation of Sulphide Minerals and Ores Table 10.4 Batch tests of induced flotation separation results .of copper-sulphur ore (Wang et al., 1991a,b,c,d) Ore type

Flowsheet in Fig. 10.10

Porphyry copperore

Disseminated copperore

Cu grade (%)

Cu recovery

Feed

Roughercone,

(%)

I

0.53

7.23

90.75

II

0.53

6.66

90.76

I

1.55

15.05

93.21

II

1.63

17.25

91.68

I

5.33

19.41

97.23

II

5.22

17.64

97.82

I

3.45

22.95

95.3

II

3.45

22.14

95.2

Figure 10.11 presents the schematic flowsheets of potential controlled flotation separation to recover chalcopyrite and pyrite from a copper-sulphur ore. Flowsheet I is collectorless flotation of chalcopyrite and then collector floatation of pyrite. Flowsheet II is collectorless flotation of chalcopyrite and then sodium, sulphide-induced flotation of pyrite. Batch flotation results are illustrated in Table 10.5. It is evident that both flowsheets are suitable for flotation separation of copper-sulphur ore. The feed ore assayed 0.38% Cu and about 6% S, the copper concentrate obtained assayed 18%- 19% Cu with a recovery of 89%. For sulphur concentrate, the grade is 37%- 43% S with a recovery of 82% - 85%. Interestingly, the grade of sulphur concentrate is higher using sodium sulphide induced flotation than collector flotation. It is further substantiated by the pilot test results for a copper sulphide ore (see Table 10.6). To maintain the pulp potential in the range of 142-189 mY, the flotation separation of copper-sulphur ore can obtain good results in collectorless or xanthate flotation system. The grade of feed ore is about 3.5% Cu, the grade of the obtained concentrate can reach to 27% - 28%Cu and recovery is above 95%. Table 10.5 Induced-flotation separation resultsof copper/sulphur ore Grade (%)

Yield (%)

Cu

S

I

II

I

Cu cone,

1.75

1.91

19.51

S cone,

13.57 12.06 0.15

Tailing

84.68 86.03 0.026 0.021

Flowsheet (%)

Feed

100

100

0.38

Recovery (%)

II

I

Cu

II

I

S

II

I

18.31 25.54 25.67 88.95 89.24 7.48

II 7.62

0.20 37.74 43.91

5.32

6.13 85.71 82.33

0.48

0.76

5.73

4.63

6.81

10.15

5.98

6.44

100

100

100

100

0.38

255

Chapter 10 Electrochemical FlotationSeparationof SulphideMinerals Feed ore

Feed ore

CaO: 3 kg/t Pine oil: 48 g/t PGE: 62g/t pH=11.5

CaO: 3 kg/t Pine oil: 48 g/t PGE: 34 g/t pH=11.5

EPt=O.18 V

Ept =O.18V

Collectorless flotation of chalcopyrite

Collectorless flotation of chalco rite

Cu cone.

Na2S: 1500g/t Pine oil: 24 g/t PGE: 25.5 g/t pH=11.0 E pt =O.095 V

Cu cone.

Sodium sulphide flotation of pyrite

S cone.

I

S cone,

Tailing

I

I

Tailing

II

Figure 10.11 Principal flowsheets of potential controlled flotation separation of copper/sulphur ore Table 10.6 Pilot plant test results of potential controlled-flotation of copper sulphideore Cu grade (%)

Flowsheet

Reagents(g/t)

Cu recovery

Feed

Cone.

(%)

Collectorless flotation

3.85

29.33

95.88

0

2.6

163

142- 189

Xanthateflotation

3.53

27.85

95.47

44.9

1.8

118

142- 189

BX

CaO

Pine oil E Pt (mY)

3. Chalcopyrite/Arsenopyrite Table 10.7 shows the flotation separation results of a mixture of chalcopyrite and arsenopyrite. It may be seen that at high pH, the chalcopyrite can be separated effectively from arsenopyrite by controlling the potential at about 280 m\Z The floated product assays 29.4% Cu and recovery is 93.48%. Table 10.7 Separation resultsof the mixtureof chalcopyrite and arsenopyrite (1: 1) by butyl xanthate(1.5 x 10- 5 mol/L) inducedflotation Products

Grade (%)

Recovery (%)

Cu

As

Cu

As

Floated

29.4

3.80

93.48

8.52

Unfloated

2.06

42.42

6.52

92.48

256

CaOpH

Eh (mY)

11

280

10.2 Flotation Separationof SulphideMineralsand Ores

10.2.2 Lead-Zinc-Iron-Sulphide Minerals and Ores Under adequate reducing environment dixanthogen will not be formed on the pyrite surface but lead xanthate is still formed on the galena surface, which underlies the basis of potential controlled flotation separation of galena from pyrite by xanthate flotation. The results in Table 10.8 show that the selective separation of galena from pyrite may be accomplished in alkaline medium and reducing potential. For the 1:1 mixture of galena and pyrite, the floated product assayed 76.31% Pb with a recovery of 90% at pH= 10.5 and potential -210 mV Similar results can be achieved for the flotation separation of a mixture of galena and arsenopyrite as shown in Table 10.9. The floated product assayed 74.36% Pb with a recovery of 92% at pH = 9.5 and potential-300 mV Table 10.8 Separation results of the mixtureof galenaand pyrite (1 : 1) by collector flotation (BX: 1.5 x 10- 5 mol/L) Recovery (%)

Grade (%)

Products

CaOpH

Pb

S

Pb

S

Floated

76.31

5.26

90.01

11.98

Unfloated

7.69

39.40

8.99

88.42

10.5

Na2S204

s, (mV) -210

Table 10.9 Separation results of a mixture of galena and arsenopyrite (1: 1) by collector-inducedflotation Grade (%)

Products

Recovery (%)

Pb

As

Pb

As

Floated

74.36

6.43

92.21

14.86

Unfloated

6.95

40.76

7.79

85.14

CaOpH

Na2S204 Eh(mV)

9.5

-300

Table 10.10 Separation results of the mixtures of two minerals (1: 1) among galena, sphalerite and pyrite by Eh controlled flotation (pH modified by lime, grinding time for 5 min, in Fe medium) Mixtures Products PbS-ZnS ZnS-FeS2

Grade (%) Pb

Zn

Pb

74.4

3.2

Zn

8.87

57.51

Recovery (%) S

Pb

Zn

88.14

4.7

S

Collector (mol/L)

pH

Eh (mV)

12 DDTC: 10-4I - - - - 90 12

11.86 95.3

Zn

58.1

31.68

92.1 40.17

S

5.13

48.56

7.9

12 BX: 10-4 12 59.83

150

257

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals

For the flotation separation of the mixtures of galena-sphalerite, using DDTC as a collector and lime as a pH modifier, the mixture is ground in Fe medium and the potential is made at 90 mV. The lead concentrate assayed of 74.4% Pb and zinc concentrate assayed of 57.51 % Zn can be obtained. The recovery of lead and zinc are, respectively, 88.14% and 95.3%. The flotation separation of a mixture of sphalerite and pyrite is conducted at pH = 12 modified by lime and using butyl xanthate as a collector. The mixture is ground in Fe medium and the potential is 150 mv, Thezincconcentrate assayed of 58.1% Zn andsulphur concentrate assayed of 48.56% S can be obtained. The recovery of zinc and sulphur are, respectively, 92.1% and 59.83%.

10.3

Applications of Potential Control Flotation in Industrial Practice

The investigations on electrochemistry related to sulphide mineral flotation have been widely reported for a long time. However, pilot tests and industrial plant operations of potential control flotation (PCF) of sulphide ores have made little progress due to the absence of an applicable control potential method. The addition of oxidants and reductants will result in a big consumption of the chemicals. In the meantime, adjusting the pulp potential by external field is also of low efficiency. Through many year's efforts, a new method has been developed in our group and applied successfully in several concentrators of galena-sphalerite-pyrite multisulphide ore. The key technological points are discussed as below.

10.3.1 Original Potential in Grinding Process Pulp potential in grinding process, is controlled by many factors, such as oxygen dissolved in pulp, oxidation of iron fine powders from the grinding media and particles of sulphide minerals, whose electrochemical interactions are verycomplex in the grinding-flotation systemof sulphide minerals ore (Nakazawa and Iwasaki, 1985). Taking galena, sphalerite and pyrite system as an example, anodic reactions include (Gu et aI., 2000): PbS+2H 2 0 = H P b O ; +S+3H+ +2e-

(10-4)

EO = 1.182(V) ZnS+ 6H20 = EO = 0.635(V) 258

ZnO; + SO~- + 12H+ + Se

(10-5)

10.3 Applications of Potential Control Flotation in Industrial Practice

FeS2 + I1H 20 =

Fe(OH)3 + 2S0~- + 19H+ + 15e-

(10-6)

EO = 0.402(V) Cathodic reaction is usually the reduction of O2 in pulp:

.!..02 +H 20+2e-=20H 2 EO

(10-7)

=0.401(V)

Moreover, due to the rise in the electrode potential difference galvanic interaction between galena, sphalerite and pyrite, it will further accelerate the oxidation of the sulphides. In addition, the following anodic oxidation reactions of iron from steel lining and balls can occur: (10-8)

EO = -0.44(V) Fe 2+ + 3H 2 0 =

EO

Fe(OH)3 + 3H+ + e

(10-9)

= 1.042(V)

These reactions will consume oxygen (02), resulting in lower pulp potential value. The mixed potential of the above reactions is originated from intrinsic electrochemical behavior of sulphide minerals in grinding-flotation systems, so it is called "original potential". The magnitude of "original potential" is lower when measured in overflow of grinding due to grinding usually in iron medium and results in an reduction atmosphere at the initial stage of flotation, which is suitable for the flotation of galena. However, the potential will increase gradually with the time due to the action of oxyge~ in the open system and make the potential unstable and flotation of galena worse. Therefore, the stable and control, of the potential is very important for flotation separation of poly-sulphide minerals. The process using the original potential to improve the separation of sulphide minerals is named "original potential control flotation" (OPCF).

10.3.2

Effect of Lime Dosage on "Original Potential"

The pulp potential is related to pH of the flotation pulp. In grinding and flotation process of PbS-ZnS-FeS2 ore, the pulp potential decreases with the increase of pH. For Nanjing concentrator of China, when the pulp pH reaches to 12.5 - 12.8, the original pulp potential range is about 0.10- 0.20 V. But for the concentrator located in high altitude area, since saturated oxygen is low, the original potential will decrease to low range from -0.06 - -0.10 V. Figure 10.12 shows the pulp pH and pulp potential in Xitieshan concentrator in Qinghai Province of China where 259

Chapter 10 Electrochemical FlotationSeparation of SulphideMinerals

the altitudewas more than 3000 m. It can be seen that, the originalpulp potential is less than -0.095 V when pH is modified to 12.8 by slime, which is the better potential region for flotation of galena. Figure 10.13 further shows that the pulp pH and pulp potential in galena flotation process can maintain stable under a higher dosage of lime, which providesthe basis for originalpotentialcontrolflotation. 10

o

GJ -10 :c: -20 r:/J ;; -30 5, -40 ~ -50 E Eo -60

6:

-70

~ -80 -90

-100

L...-.....L.--L----L-...J--''-----'---''-----L--'-----''--'----L...--'--..L...--L--'----L...-~

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 pH

Figure 10.12 Pulppotential as a function of pH in Xitieshan flotation concentrator 0.24....------------------, 0.22

G3' ::r: tr:

Lime:

~ 10 kg/t ........ 6 kg/t

0.20

>"'

~ 0.18

0.16

..... •

...... 2

468

• 10

Rougher flotation time(min)

Figure 10.13 Effectof limedosage on the pilippotential in galena rougher flotation process ofNanjing concentrator

10.3.3 Coupling with Other Flotation Process Factors In the open system, the pulp potential after grinding will increase with the process of aeration of agitation and flotation (see Fig. 10.14). Because the flotation of galenaneedsrelatively low potential in whichleadxanthate is formed easilyon the galena surface. Therefore, the flotation operational time mustcouple withthe change 260

10.3

Applications of Potential Control Flotation in Industrial Practice

of the pulp potential so as to selectively separate galena from other minerals requiring relatively high potential for flotation like pyrite. Table 10.11 lists oxygen dissolved and pulp potential in the grinding-flotation system tested in Beishan flotation concentrator. The results indicated that the dissolved oxygen and potential in mill charge and classifier overflow are lower, and increased with the flotation time. It is advisable for the flotation time of lead circuit to be less than 18 min. Table 10.11 Oxygen dissolved and the pulp potential in Beishan concentrator of PbS-ZnS ore O2 conc. (%)

E(V, SHE)

Mill discharge

-

3

0.147

Classifier overflow

-

9

0.156

Test sites

Time (min)

Stirrer

4

30

0.166

PbS flotation Rougher

8

35

0.174

Scavenger I

4

60

0.181

Scavenger II

3

74

0.190

Scavenger III

3

92

0.198

60.--------------------. 40

::cir:W

20

>='

5

]

0 -20

= ~ -40 0..

0..

~ -

60

-80 -1 00 .....-......&...._£..~~_'__'___L.._L.___L__J~_'___'_......&...._£..___L__J'___'__.L.._..I._......&...._£..___L__J o 5 10 15 20 25 30 35 40 45 50 55 60 65 Flotation time (min)

Figure 10.14 Change of the pulp potential with the operation time in Xitieshan flotation concentrator

10.3.4 Coupling with Reagent Schemes Lower original potential in grinding is beneficial to flotation selectivity of galena. Thus, collectors should be added into the mill directly. Table 10.12 is the results of flotation separation of Beishan concentrator of PbS-ZnS ore for collector addition in different places. It shows that the recovery and grade of galena concentrate are improved obviously when collectors are added in mill compared to in agitator. 261

Chapter 10 Table 10.12

Electrochemical Flotation Separation of Sulphide Minerals

The flotation results in Beishan concentrator ofPbS-ZnS ore PbS cone,

Collector adding site

Agitator

Mill

Grade (%)

61.13

64.23

Recovery (%)

74.91

76.07

10.3.5 Coupling with Flotation Circuit Under suitable pulp potential in theOPCF process, the flotation speed of galena is very fast and most galena has been floated just in rougher flotation stage. So the flotation cell of galena scavenger flotation operation can be diminished and the flotation circuit is further optimized. The comparison of operation parameters between traditional flowsheet and OPCF flotation is presented in Table 10.13. As can be seen, the flotation potential and time are lower in OPCF than in traditional flowsheet. Table 10.13 Comparison of the operation factors between traditional flotation and OPCF flotation

PbS rougher flotation

PbS cleaner flotation

Traditional flotation

OPCF flotation

pH

8-9

12.5 - 12.8

Eh(mV)

350 - 280

180 - 150

Time (min)

20

8

pH

8.0

12.0

s, (mV)

350

195

Time (min)

20

10

10.3.6 Applications of OPCF Technology in Several Flotation Concentrators 1. Nanjing Lead-Zinc Concentrator Figure 10.15 is the OPCF technology used in the flotation concentrator ofNanjing lead-zinc mine and the operation results of the OPCF technology is shown in Table 10.14. The OPCF separation of galena from sphalerite and pyrite is conducted in the strongly alkaline pulps ofpH= 12.4-12.5 modified by lime, with sodium diethyl dithiocarbamate (DDTC) as a collector and without the addition of ZnS04 and Na2S03 as the depressants of ZnS and FeS2.

262

10.3 Applications of Potential Control Flotation in Industrial Practice

It can be seen from Table 10.14 that in common flow sheet a depressants is used and pH is neutral, the flotation separation of galena, sphalerite and pyrite is not so good. The OPCF can much improve the results of selective flotation of PbS-ZnS. Compared with common flowsheet, the grade of concentration increases respectively from 52.1 % to 60.0% for Pb, from 52.6% to 53.0% for Zn and from 37.4% to 46.5% for S and the recovery of concentration increases respectively from 85.9%to 88.9%for Pb, 87.0%to 91.9%for Zn and 68.8%to 71.9%for S.

Feed

Tailings

FeS cone.

Overflow water

Figure 10.15 OPCFprocess in concentrator ofNanjing PbS-ZnS-FeS2 ore

Table 10.14 Comparison of the flotation results in concentrator of Nanjing PbS-ZnS-FeS2 ore Flowsheet

Traditional technology

OPCF technology

Products

Pb

Grade (%) Zn S

Pb

Recovery (%) Zn S

Pb cone.

52.1

5.59

20.8

85.9

4.27

Zn cone,

1.23

52.6

32.4

4.41

87.0

S conca

0.61

1.38

37.4

6.91

7.20

Pb cone.

60.0

4.68

17.4

88.9

3.47

Zn conca

1.03

53.0

30.6

3.58

91.9

S cone.

0.55

0.60

46.5

5.16

2.80

Conditions for PbS flotation

5.61 pH= 7.0 DDTP: 40 g/t 18.95 Frother: 20 g/t ZnS04: 1.1 kg/t 68.80 Na2S03: 0.7 kg/t 4.22 pH= 12.4 Eh = 170mV 17.42 Lime: 5 kg/t DDTC: 60 g/t 71.9 Frother:20 g/t

2. Xitieshan Lead-Zinc Concentrator

The industrial test of OPCF technology is employed in Xitieshan lead-zinc mine, which is locatedin Qinghai Province of China with high altitude above sea level. Figure 10.16 shows the flowsheets ofOPCF. The effect of pulppotential on flotation separation of galena and sphalerite in industrial test .can be seen in Fig. 10.17. It follows that the suitable pulp potentialrange of galena flotation is located in less than -5 m~ at where the floatability of sphalerite is very poor. The sphalerite 263

Chapter 10 Electrochemical Flotation Separation of Sulphide Minerals pH 1= 12 -13 Eh=-0.06 - 0.08

Lime

Lime CuS04 Xanthate pH2

Feed

PbS conc. ~---~Tailings

ZnS cone.

Figure 10.16

OPCF flowsheet of concentrator in Xitieshan PbS-ZnS-FeS2 ore

flotation begins at the potential range of more than 5 mV, when the potential reaches to 60 mV, the recovery of sphalerite is more than 90%. 110 100 90

-.....Pb

-+-Zn

•

80 ~

70

'6

60

0

50

'$Q)

>

oQ)

"

40 30 20 10

o~

•

-90-80-70-60-50-40-30-20-10 0 10 20 30 40 50 60 70 80 Pulp potentail (mV, SHE)

Figure 10.17 Effects of pulp potential on the flotation separation of PbS-ZnS in Xitieshan concentrator

Figure 10.18 further shows the flotation separation efficiency of galena and sphalerite from the date of the quantity containing Pb or Zn each other in the corresponding concentration. It may be seen that the recovery of ZnS in PbS concentration and PbS recovery in ZnS concentration are very low, indicating that the flotation separation of PbS and ZnS is selective in this new flowsheet. The comparison of flotation results using OPCF and traditional flowsheet is shown in Table 10.15. It can be seen that either PbS, ZnS flotation recovery or the grade of the concentration, is improved obviously in OPCF. The grade and recovery of lead concentration, respectively, increase from 73.77% to 79.19% and 264

10.3 Applications of Potential Control Flotation in Industrial Practice

from 91.86% to 93.69%. The grade and recovery of zinc concentration are, respectively, enhanced from 49.63% to 49.85% and 86.44% to 91.93%. In addition, the flotation rate of galena in the OPCF process is much faster than that in the traditional through the measurement of galena recovery of each cell as shown in Fig. 10.19. The Pb recovery is more than 90% only through 4 flotation cells operation and Zn recovery reaches to 87% only through 12 flotation cells operation in OPCF. But for the traditional process, 8 flotation cells are needed to reach 90% Pb recovery and 20 flotation cells are needed to reach 87% Zn recovery. 100

80 ".-...,

'i(

'6 ~

>

60

0

C)

~ ....

(;) c

N

40

czl

.D

e:20

'E

.D 0..

N

'~

=

cd

o

ZnS flotation

t: 0

= ''SEo o

5 10 15 20 Zn recovery in PbS cone,

25 30 35 Time(min)

40

45 Pb recovery in ZnS cone,

Figure 10.18 The separation ofPbS-ZnS and flotation time by OPCF in Xitieshan

concentrator Table 10.15 Comparisons of flotation results ofOPCF and traditional flotation in concentrator of Xitieshan lead-zinc mine Flowsheet

Products

Traditional technology OPCF technology

Grade (%)

Recovery (%)

Pb

Zn

Pb

Zn

Pb cone.

73.77

2.54

91.86

4.27

Zn cone.

0.81

49.63

4.41

86.44

Pb cone.

79.19

2.10

93.69

3.47

Zn cone.

0.52

49.85

3.58

91.93

Conditions for PbS flotation pH= 7.0- 8.0 Frother: 130g/t Collector: 130 g/t pH = 12.0- 13.0 Eh = -0.08 - -0.06 V Lime: 5 - 7 kg/t DDTC: 28 - 35 g/t 265

Chapter 10 Electrochemical FlotationSeparation of Sulphide Minerals 50 r - - - - - - - - - - - - - - - - - - - - - - - , 45.5 EZZ] Ph recovery ~ Zn recovery 40

10

No.1No.2No.3No.4No.5No.6No.7No.8No.9 No.10 No.12 No.14 No.n No.13 Flotation cell

Figure 10.19 The flotation rate (the recovery of each cell) for OPCF in Xitieshan concentrator

3. Fankou Lead-Zinc Concentrator

The flotation separation of galena, sphalerite and pyrite in Fankoulead-zinc mine is very complicated because these three minerals are finely disseminated. The OPCF technology is also successfully appliedto this plant to separate these three minerals. Here, pH is modified to 12 by lime and pulp potential is maintained as less than 170mV. The mixture of xanthate and DDTC is used as a collector in flotation of galena. CUS04 is used as a collectorin the flotation of sphalerite. The principal flowsheet of OPCF for flotation separation of Fankou lead-zinc ore is given in Fig. 10.20. The comparison of results of plant production for OPCF and old flowsheet is listed in Table 10.16. It can be seen that the OPCF technique Table 10.16 The comparison of resultsof plantproduction for OPCFand old flowsheet Products

266

Recovery (%)

Grade (%) Pb

Zn

Pb

Zn

Pb cone.

58.02

3.97

82.4

2.54

Zn cone.

1.29

53.03

4.98

92.27

Tailings

0.66

0.59

16.62

5.19

Feed

4.25

9.43

100

100

Pb cone.

61.44

2.56

87.66

1.61

Zn cone.

0.83

56.34

3.16

94.62

Tailings

0.61

0.57

9.18

3.77

Feed

4.25

9.43

100

100

Flowsheet

Old flowsheet

OPCF technology

10.3 Applications of Potential Control Flotation in Industrial Practice Feed ore

1: Q) E

e::::s ~

Q)

E

.s1: s

Ph scavenger

~ 1________ e

,

I

I

Zn cleaner Zn cone.

Ph cone.

,

'

Zn seavenger Zn tailings

Figure 10.20 The principal flowsheet ofOPCF for flotation separation of Fankou

lead-zinc ore

enhances greatly the efficiency of flotation separation of lead-zinc in Fankou mine. The grade and recovery of lead concentration are, respectively, increased from 58.02% to 61.44% and from 82.4% to 87.66%. The grade and recovery of zinc concentration are, respectively, enhanced from 53.03% to 56.34% and 92.27% to 94.62%. Moreover, the reagent consumption is diminished in OPCF relatively to old flowsheet as given in Table 10.17. It not only decreases the dosage of collector and CaO, but also cancels the ZnS04. The total volume of flotation cell required for flotation of galena descends due to the rapid flotation rate of galena in OPCF and hence the numberof cell is abbreviated. Table 10.17 The comparison of reagentconsumption plant production for OPCF and old flowsheet CUS04

ZnS04

DS

EX

DDTC

CaO

2#oil

Old

0.61

0.35

0.079

0.102

0.062

8.8

0.128

OPCF

0.65

0

0.02

0.08

0.04

8

0.128

Flowsheet

267

Chapter 10

Electrochemical Flotation Separation of Sulphide Minerals

4. Tonglushan Concentrator of CuFeS2-FeS2 Ore Application of Collectorless Flotation Figure 10.21 presents the flow sheet for the collectorless flotation and separation of chalcopyrite and pyrite in Tonglushan copper mine in China. The pulp potential is 0.175 V and pH is 11.5. The industrial test shows that the collectorless flotation can obtain better results than the traditional collector flotation. Feed

Cu cone.

Pyrite cone.

Figure 10.21 Flow sheet of collectorless flotation for concentrators of Tonglushan CuFeS2-FeS2ore

268

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References Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing, 2004d. Electrochemistry of copper activation of marmatite. Metal Mine, (2) (in Chinese) Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing. 2004e. Interface electrochemistry of interaction of collector with jamesonite. The Chinese Journal of Nonferrous Metals, 14(1): 127-131 (in Chinese) Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing, 2004f. Study of electrochemistry of jamesonite in DDTC and saturation Ca(OH)2. The Chinese Journal of Nonferrous Metals, 14(10): 1763 - 1769 (in Chinese) Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing, 2004g. The corrosive electrochemical study of marmatite. Journal of Chinese Society For Corrosion and Protection, 24(4): 226 - 229 Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing, 2005. Electrochemical adsorption behaviour and mechanism of diethyldithiocarbamate on surface of marmatite. The Chinese Journal of Nonferrous Metals, 15(9): 1452 - 1457 (in Chinese) Yu Runlan, Qiu Guanzhou, Hu Yuehua, Qin Wenqing, 2006. Study on mechanism of diethyldithiocarbamate with jamesonite by spectroelectro-chemistty. Mining and Metallurgical Engineering, 26(1): 29 - 31 (in Chinese) Zhang, D.H., 1996. Adsorption and photodesorption of oxygen on the surface and crystallite interfaces of sputtered ZnO films. Materials Chemistry and Physics, 45(3): 248 - 252 Zhang Qin, Hu Yuehua, Gu Guohua, Xu Jing, 2004a. The study on the Interaction between ethy I xanthate and pyrrhotite in electrochemical flotation by FTIR spectroscopy. Mining and Metallurgical Engineering, 24(5): 42 - 44 (in Chinese) Zhang Qin, Hu Yuehua, Gu Guohua, Xu Jing, 2004b. Selective flotation separation ofjamesonite from pyrrhotite by potassium cyanide. Journal of Central South University of Technology, 35(3): 372 - 375 Zhang Qin, Hu Yuehua, Gu Guohua, Nie Zhenyuan, 2004c. Study of the pyrrhotite self-induced flotation electrochemical. Nonferrous Metals, (2): 4 - 6 (in Chinese) Zhang Qin, Hu Yuehua, Gu Guohua, Nie Zhenyuan, 2004d. Electrochemical flotation of ethyl xanthate-pyrrhotite system. Trans. Nonferrous Met. Soc. China, 14 (6): 1174 - 1179 Zhang Qin, Hu Yuehua, Gu Guohua, Xu Jing, 2004e. Selective flotation separation ofjamesonite from pyrrhotite by lime. Mining and Metallurgical Engineering, 24(2): 30 - 32 (in Chinese) Zhang Qin, Hu Yuehua, Gu Guohua, Xu Jing, 2004f. Mechanism of Cu2+ ion activation flotation of marmatite in absence and presence of ethyl xanthate. The Chinese Journal of Nonferrous Metals, 14(4): 676 - 679 (in Chinese) Zhang Qin, Xu Jing, Hu Yuehua, Chen Tiejun, 2006. Study on separation of jamesonite from pyrrhotite by using RC as a new inhibitor. Mining and Metallurgical Engineering, 26(1): 27 - 28 (in Chinese) Zhao Jing et aI., 1988. Research on the mechanism of chalcopyrite depressed by sodium mercaptoacetic. Nonferrous Metals (part of mineral processing), (3): 42 - 45 Zhuo Chen and Yoon, R. H., 2000. Electrochemistry of copper activation of sphalerite. Inter. 1. Miner. Process, 58: 57 - 66

285

Index of Terms

A absolute temperature 29 activator 112,127,142,143,146,159,160,161,163-165 active point 213 adsorption isotherm 133 adsorption mechanism 2,112 affinity 163,164,221 alkylxanthate 11 ammonium dialkyl dithiophosphate (ADDP) 76 ammonium persulphate 245 anodic current 9-11,41,42,45,46,52,59,67,68,74,75,76,90,117,165,165,208,212 anodic oxidation 2,8,41,42,46,47,86,116,117,259 anodic polarization curve 18 aqueous solution 35,37,41,67,99,142,144,282 argentite 4,62 arsenopyrite 4,5,7,10,15,28,35-38,47,48,53,58,59,61,62,64,90,91,93,124,127,129,130-141, 147-149,152-159,249,250,256,257,270,272,276,280,284 auger electron spectroscopy 152 autogenously grinding 15,272

B band model 219 band theory 13,19,271 binding energy 48,109,110 bonding strength 163 buffer solution 43,45,46,59,74,86,87,90,91,150,151 butyl xanthate (BX) 64,68,71,73,82,86,92,93,116,117,126,127,129-133,138,139,142,143, 147-149,152-154,157-159,208,211,212,238,239,242,245,247,251,256

C Cambridge serial total energy package (CASTEP) 221 capacitance impedance loop 79 capacitive reactance 171,173-180,182,183,185,186,188,190-192,194 cathodic polarization curve 18

Index of Terms cathodic reduction 2,3,7,8 ceramic mill 73,201,251,253 chalcocite 7,10,63,65,67,68,92,94,95,100,146 chalcopyrite 3,4,7,10-12,15-17,19,23,28,30-35,41,42,49,53,54,57,61,62,64,67-69,93,99,100, 124,125,244-251,253-256,268 characteristic absorption band 100,102,103,105,106 characteristic absorption peak 102 characteristic index 139 chemistry corrosion 167 cleavage surface 222,230 coarse grinding 215 collector/water/mineral system 91 collectorless floatability 2-4,6,23,29,35,38,40,47,54,55,57,62,238,241,248,251,254 conduction band 220,224,230,235-237,240,241,242 conduction carrier 222 contact angle 9,11,59,60 copper sulphide minerals 63,65,244,253 corrosion current 17 corrosion inhibitor 170 corrosive current 77,78,82,121,169,170,172,173,175,178,181-184,186-188,190,192,195,197 corrosive current density 170,172,173,175 corrosive Electrochemistry 79,167,172,178,181,183,186,187,190,192,193,195-199 corrosive potential 77,78,82,118-120,167,170-172,175,178,181,183,186,187,190-192,194, 195,197,198,200,203,204 corrosive speed 169,170 covalent bond 13,226,231,232 covalent radius 164 crystal chemistry 3 crystal field theory (CFT) 224 crystal lack 202 crystal structure 3,20,21,222,229 current density 168-170,172,173,175 cyclic voltammetry (CV) 20,41 cyclic voltammogram 11,45,53,59,67,74,75,90,142,144,145,146 cyclohexane

6,49,96-98

D decomposition potential 68,85 density function theory (DFT) 219 density of states (DOS) 224,230 depressant 112,114,116,118,120,124,125- 134,136-140,142,152-155,157,158,184,195,247, 262,263 dialkyl dithiocarbamate (DDTC) 73,105, 262

287

Index of Terms diethyI dithiophosphate 113-115 diethyl dixanthogen (EX2) 97 diethyl mono thiocarbonate (MTC) 97 diethyI thiocarbonate (EPX) 97 diffraction band 60 diffuse reflection infrared fourier transform spectroscopy (DRIFT) 99 diffusion layer 80,81,119 digital pressure gauge 202 dilute solution 81 dimethyl phthalate (DMPS) 126 dislocation 14,202 distilled water 15 dithiocarbamate (DDTC) 73,76,104,262 dithiolate 3,8,8,63 double electric charge layer 81,119

E EDTA 250 3,254,257 Eh-pH diagram 20,32-35,37,38,40,41,48, 53,58,91,124,149 electric charge density 81 electrical resistivity 82 electrocheimcal corrosive method 19 electrochemical adsorption 8,74,285 electrochemical Behavior 12,14,19,74,82,201,202,203,259,258 electrochemical corrosion 167,168 electrochemical equilibriumcalculation 3 electrochemical measurement 3,19,65 electrochemical phase diagram 34,59,63, 91,92,93,95 electrochemical reaction 2,7,8,14,78,79,92,94,119,123,221 electrochemical resistance 79,80 electrochemical theory 7,146

s; control

electrochemistry impedance spectrum (EIS) 79,171 electrochemical impedance 7,79,120 electrochemistry of flotation 112 electrode potential 9,14-15,29,78,168,169,174,177,205,207,210-212,247,252,259 electron transferring mechanism 219 empirical relationship 1 empty band 220 energy level diagram energy values

equilibrium constant equilibrium line 288

13

14 213,214

34,35,37,41,59,93,95

Index of Terms ethyl xanthate (EX) 9,10,12-14,64,65,67,68,71,73,76,84,85,89,90,95,97-104,109-111,114,115, 117,122,123,147,150-152,161,247,254,270,273,274,276,279,283,285

F face-centered cubic structure 20 Faraday constant 29 Fermi level 224,226,227,240 filled band 220 fine grinding 215,217 flotation behavior 5,8,12,13,19,24,25,29,32,47,52,85,92,112,129,221,238,244,245,246,251 flotation separation efficiency 264 forbidden energy gap (band gap) 220 frontier molecular orbital 14,219 FTIR reflection spectra 12,100-108,128 FTIR-ATR 11,99

G galena 5,7,10-18,20,21,23,24,28,31,34,35,42,43,48,49,51-54,57,62,63,69-76,93,99,100,109, 113,114,116,117,122,124,125,159,167,170,171,186-196,200,201,206-214,221,240,241,242, 243,247,250-254,257 -267,269-276,278-281,283,284 galvanic cell 16,19,201 galvanic interaction 16-19,201,202,244,259 gas constant 29 generalized gradient approximation (GGA) 229 grade 81,161,254-257,261-267 grinding media 14,16,17,109,201,203-212,215,244,249-252,258 grinding-flotation system 169,258,259,261 2,3,4,5,6, five hydroxyl heptyl dithio carbonic sodium (GX3) 138 2,3-dihydroxyl propyl dithiocarbonic sodium (GX2) 131 2-hydroxyl ethyl dithiocarbonic sodium (GX1) 152

H hemihedral symmetry 20 highest occupied molecular orbital (HOMO) 219 hydrogen peroxide (H20 2) 124 hydrolysis reaction 57 hydrophilic radical 125 hydrophilic-hydrophobic balance value 139 hydrophobic entity 2,20,29,30,33,35,47, 48,52,53,57,59,63,65,68,70,85-88,90,92,95,96,99, 103,113,122,167 hydroxide ions 1

I infrared spectra (IS) 2

289

Index of Terms infra-red(IR) spectroscopy 10,95 inherenthydrophobicity 3,6 interfacial capacitance 79,80 iron Sulphide Minerals 63,86,126

J jamesonite 23-27,38,40-43,49-51,53-57,63,76-82,96,97,103-104,106,108-111,117,118, 120-122

K kaliumbutyl xanthate(KBX) 68,86 kalium ethyl xanthate(KEX) 11,12,68,99

L lead sulphideminerals 63,69 lime 112,116,159-161,163,165,167,175-177,183-189,195-198,200,244,246,247,252,257-260, 262,263,265,266 linear potentialsweep voltammetry (LPSV) 20,41 localgradient-corrected exchange-correlation functional (LDA) 221 lowestunoccupied molecularorbital (LUMO) 13,219 lower limitingflotationpotential 67,68

M marcasite 62 marmatite 23,25-27,48-50,55-57,84-87,96-99,102,118-120,125-136,147-159,228,229,233-238 mechanical electrochemistry behavior 202,203 mechanical power 213,214 mechanical rubbing 215 mechano-electrochemical behavior 19,201,203 metal-collector salts 1 metastable phase 32,34,35,37,41,42,58,59 mineralcrystal 14 mineralelectrodepotential 168,169,247 mineral-thio-collector 10,63,95 mixedpotential 3,7-9,11,16,63-65,112,168,169,201,220,237,238,259 mixedpotentialmodel 3,7-9,63,64,112 molecularorbital (MO) theory 3,19,281 molybdenite 2-4,6,10,21,22,62 multi-electrode galvanic contact 18

N natural floatability 2-4,6,20,22,23,26,27,201,273-275 negativepotential 77,143 neutral solution 40

290

Index of Terms

o opposite electrode 202-211 organic anion 82 organic depressant 112,125,126-128,139,271,276,281,284 original potential 244,258,259,261,274 original potential control flotation (OPCF) 244,259,260 over potential 32,245 oxidation-reduction potential 29 oxidation-reduction reaction 168 oxidizing environment 4,10,23,73,249,250,253 oxidizing potential 28,52,69 oxy-hydroxide 16

p partial density of state (PDOS) 225,233 passivation film 174,177 perhydroxide 7 periodic boundary condition 222,230 phosphate 43,163 photoluminescence 236 physically absorbed 109 plate capacitor 81 platinum electrode 26,57 polar radical 126 polarization curve 16,18,119,167,170,171,174,177,180,182,186,189,191,194,196,197,198 polarization resistance 169,170-173,175,178,181,183,184,186-187,190,192,193,195-199 polarized curve 8 polarographic method 9 polyacrylamide polymer (PAM) 112,126 poly-metallic sulphide mineral 159 polysulphide 48 potassium permanganate 124 potential difference 80,259 potential modifier 244-246,254 potential sweep measurement 67 potential value 105,106,147,246 pretreatment 70,244,250 propanic sodium dithio carbonic sodium (TX4)

131,134,136

pulp potential 4,5,7,19,23-26,56,61,63,65,67-69,71-73,75-77,79,81-89,91,100,101,103-108,112, 124,131-133,142,147-149,155-156,244-247,249,250,255,258-264,266,269,271,279,281,284 pyrite (Py) 3-7,9,12,13,17,18,20,21,23,24,26,28,35-37,46,53,54,56,57,59-61,63,87,88,92,93, 98,99,109,113-117,122-127,160-163,165,166,170,172-186,201,203-208,211-215,217,219-224, 226,240-242,245-248,250,251,254,255,257,258,262,265,267

291

Index of Terms pyrrhotite 5,12,17-19,26,45,46,49,51,54,88-91,99,100,101,105-107,117,118,125-127,129-137, 139-141,147-149,152-155,271,274,277,281,284,285

Q quantum chemistry calculation

219

R recovery

18,22,24,25,50-51,53-57,65,68,69,71-73,76,82,84-89,91,93,95,101,102,103,105,106,

117,118,124-127,129-131,138-143,147-149,152,154-156,160,161,245-250,253-258,261-264, 266,267 redox potential 3,29,245,273 reducing environment 207,246,249,251,257 reference electrode 202 rest potential (mixed potential) 9,10,15,16,62-64,92,146,201,207,269 reversible potential 9-11,63,74,88,91,100,145,182 rotating disc electrode technique 7

s self-induced collectorless flotation 5,37,53-55,57,62 self-induced flotability 4,5,30,33,35,51,62,250,269,270,281,285 SEM 48 semiconductor energy band theory

19

semiconductor property 12,13 sodium acetate dithio carbonic sodium (TX1)

138

sodium cyanide 112 sodium hypochlorite (NaCIO) 124 sodium propronate dithiocarbonic sodium (TX2) 138 sodium sulphide 53,54,56-58,60-62,73,112,122,123,245-247,255,277,279 sodium tetraborate 11 solution chemistry 142,161,282 specific adsorption 179 sphalerite 7,14,15,17,18,23, 44,45,82-85,99,126,142-146,167,170,197-201,204,205,209-212, 217-219,228-230,236,237,252,258,262,262,263,274,275,278,282,284 stainless steel

17,18,109,251

standard free energy

28,29,66,70,71,114

standard hydrogen electrode

29

standard hydrogen electrode (SHE) 29 static potential stibnite

168,169,207,213

2,4,21,22,62,249

stoichiometric

18,109,279

stretching vibration strong acidic medium

292

100,103,128,129 26

Index ofTenns strong alkaline medium 26,51 structural feature 6,20 sulfhydryl acetic acid

112,125

sulphide minerals 1-5,7-14,19,20,23,25,26,28-30,32,41,48,50,52,53,57,62,63,65,112,136,137, 149,152-155,157,167,171,172,201,202,215,217,220,237,238,244,245,249,250 sulphur-induced collectorless flotation 5,54,55,56,62 sulphur-induced flotability 6,61,246 surface analysis 2,3,19,20,48,49,51,53,60,61,95,97,99,101,103,105,107,109,111,112,142,150, 165 surface coverage 48,61,109 surface electron structure

13

surface pretreatment 250 surface sensitive spectroscopic technique

10,95

T Tafel slope 77,78,213,214 tetrahedral configuration 20 thermodynamic calculation 65,279 thermodynamic equilibrium 91,281 thio-collectors

1-3,65,99,237

thiosulphate 7,29,32-38,73 tight layer

81,120

transmission factor two-electrode system

169 16

(I-carbonic sodium-2-hydroxyl) sodium propronate dithio carbonic sodium (TX3)

138,154

u ultra violet (UV) sepctrophotometer 96 ultrasoft pseudo potential 221,229 upper flotation edge 68 upper limiting potential 95 UV spectra photometer 6

v valence band 220,224,225,230,234-236,240,241 valence electron 164 van der waals theory 3 voltammograms 10,11,42,43,45-47,68,69,71,73-76,79,82,86-88,90,108, 116,117,151

W wave function 221,229

293

Index ofTenns

x xanthate

1-3,7,9-15,17,18,63-73,76,82-86,88-104,109-112,114-117, 122-133,135,136-143,

147-149,151-159,161,178-186,190-191,192,194-196,198-201,203-205,207-209,211-214, 229,237-240,242,245-247 ,251,254-258,260,269,270,273,274,276-283 X-ray diffraction diagram

61

X-ray photoelectron spectroscopy(XPS)

2,10,48,61,95,109,270,272,275,277,278

y yield

161,236,255

z zeta potential

136,137,142,157-159

zinc Sulphide Minerals

63,82

zinc-iron sulphide mineral

294

136,137,146,147,149,151-155,157,197,244

Index of Scholars

A Abramov A A 92,269 Abrants L M 272 Adam K 15,16,269 Agar G E 45,275 Ahlberg E 7,221,227,269 Ahmed S M 2,8,70,269 AhnJH 269 Aldred W 274 Allison SA 3,8-10,68,70,90,269 Aloson F N 269 Andrea P G 269 Andrew H 221,225,269 Angela G L 269 Antti B M 273 Aplan E F 272 Arce E M 2,269 Asare K 0 283 Atak S 2,270 Athryn E P K 269 Avdohin V A 269 Avotins P V 277

B Bae I 274 Bai Shibin 282 BaldaufH 126,270 Ball B 88,123,270 Bancroft G M 277 Basilio C I 275,276,283 Basiollio C 65,95,270 Bauer D 276 Beattie M J V 47,124,270 Beattie D A 47,124

Index of Scholars Bechtold E 227,270 BeckerU 282 Bessonov S V 2,12,278 Biegler T 7,41,270 Botelho do Rego A M 272 Briceno A 271 Brinen J S 277 BrionD 7 Broo A E 7,221,227,269 BrownGM 283 Buckley AN 2,3,8,48,270,277 Bulut G 2,270

c Cao Limei 272 Cao Xuefeng 276,281 Caroline S 272 Cases J M 17,109,251,270,271,277 Castellanos R H 278 Castillo R 277 Cata M 13,271 Chander S 3,4,32,41,47,92,271,272,278 Charnock J M 278 Chen Jin 271,272 Chen Tiejun 285 Cheng Jianhua 271 Cheng X (X Cheng or Xuan Cheng) 5,13,15,19,23,69,125,126,271,284 Cheng W 5,13,15,19,23,69,125,126,271 ChemyakA S 47,276 Chemyshova I V 240 Christ C L 29,30,32,70,273 Chung-Liang Chang 271 Ciccu R 271 CookMA 2,271 Costa M C 2,272 CoxAB 2,122,123,283 Craig J R 4,21,22,29,30,32,70,282 CreedMD 282

D Dai Jingping 272,275 David R Lide 230,272 de Donato P 270,271 Delfa G 271

296

Index of Scholars DemirelH 5,272 Ding Dunghuang 272 DingleyW 275 Domianowski A 276 Dong Qing-hay 272 Dooley R K 278 DowlingE 277 Du Rietz 1 Du Tietz C 66,70,272 Dunn HowardJ 47 DuunJG 272

E EadingtonP 12,272 EdelbroR 221,230,272 EkmekciZ 5,272 ElgillaniD A 273 EliseevN I 61,272 EnglandK E R 275,278,282 FormanekV 14,201,279 Erre R 270 EschrigH 278 EspositoM C 12,272

F Fagremo0 272 FahlstromP H 15,272 Feng Jimin 271 Feng Qiming 271,272 FereshtehR 272 FernandezP G 272 Fierro R E 221,272 FinkelsteinN P 2,10,22,65,70,182,229,269,273,274,280 Ford C E 276 FornasieroD 98,273 Forssberg K S E 92,250,270,271,273,274,283 Fozo S 278 FreemanC 275 FuerstenauD W 2,3,7,8,92,116,124,247,271 FuerstenauMe 2,3,7,8,92,116,124,247,271-273,278,279 Fukui Ken-ichi 280

G GardnerJ R 5,9,11,23,41,70,88,273 GarrelsR M 29,30,32,70,273

297

Index of Scholars Gaudin A M 1,3,7,65,270,273,278-280 Gebhardt J E 269,273 Gerson A R 278 Gjerdrum A S 272 Glazyrina L N 272 Gonzalez I 2,269 Goold LA 8,10,22,182,269,273 Goto Yoshio 280 Grano S 10,95,251,273 Grano S R 10,95,251,277 Granville A 269 Gu Guohua 74,274,285 Gupta S 221,274 Guy P J 4,10,11,14,23,31,35,41,57,73,274

H Hamilton I C 42,45,46,270,274 Harris P J 15,65,81,274 Haydon B C 284 Hayes R A 2,3,5,23,62,274 Hazell L B 10,95,278 He Zhiguo 284 Hepel T 32,66,92,274 Heyes G W 2,5,6,26,45,46,47,49,56,59,60,65,67,68,95,245,248,251,274,282 Hiskey J Brent 47,48,280 Hodgson M 45,275 Hoey G R 14,275 Hope GA 283 Home M D 41,270 Hoyack M E 124,275 Hope GA 283 Hu Q 2,8,13,15,29,30,92,116,280 Hu Yuehua 272,274-277,279,281-285 Huang Kaiguo 282 Huang K 126,283 Hughes H C 272 Hung A 225,277

I Irene Y 269 Iwasaki I 5,15-17,19,23,69,244,251,258,269,271,276-279,284 Iwasawa Yasuhiro 280

J JamesAF

298

272

Index of Scholars Janetski N D 9,88,115,123,124,275 Jellinek F 230,275 Jiang Hao 275 Jiang Yuren 276 Jin H 276 Johnson N W 32,275 JosephM 269

K Karkovsky I A 1,114,275 Kar G 271 Kartio I J 275 Kelebek S 49,275 Kelsall G H 7,41,275,276,282 KimD S 283 King F 221,276 Kirbitova N V 272 Kitchener J A 3,7,57,70,280,281,284 Kneer E A 202,276 Koepemik K 278 Kongolo M 270 Kostina G M 47,276 KowalA 67,276 KuhnMC 273 Kurscakova G M 269

L Laajalehto K 10,95,109,276 Labonte G 26,276 LamO 276 Lamache M 276 Learmont ME 15,16,276 Leja J 279 Leonov S B 269 Lepetic V M 3,276 Leppinen J 0 10-12,95,99,100,102,103,276 Li Baidan 280-282 Li Haipu 276 Li Shikun 271 Li Y Q 15,126,280 Lin Q 125,275 Ling H G 271 Litke C D 275 Liu Q 125,276 Liu Hui 284 299

Index of Scholars Liu Runqing 276,281,284 Liu Ruyi 274 LombeWC 283 Long Xiangyun 272,281,282 Lovell V M 273 Lowson R T 47,276 Lu Yiping 271 Lusk J 228,276 Luttrell G H 48,61,276,277

M Ma Guoyin 277 Majima H 7,9,88,277 Malicsi A S 17,279 Malmensten B 278 Matsuoka I 280 Maust E E Jr 3,280 McCarron J J 61,277 McClure J C 278 Miaw HL 273 Michot L 270 Mielezarski E 10,95,100,102 Mielezarski J A 10,95,100,102,277 Mishra K 12,277 MisraM 273 Montalti M 273 MoonK S 279 Moreira J 278 MoryM S 277 Mosselmans J F W 278 Munro P D 32,275 Muscat J 221,277

N Nagaraj DR 124,126,277 Nakazawa H 15,17,244,251,258,277 Natarajan KA 16-18,244,251,269,277,282,284 Neeraj K M 7,277 Nesbitt H W 222,277 NicdM J 269 Nie Zhenyuan 285 Nixon J C 2,271,277,280 NowakP 276

300

Index of Scholars

o O'DeaAR 278 Ocampo A L 278 O'Dell C S 65,67,278 O'M Bockris J 278 Opahle I 221,278

p Page P W 10,41,95,275,278 Palmer B R 273 Palsson B 273 Pang J 41,271,278 Pattabi M 221,278 Pattrick R A D 237,278 PaulJ 272 Persson I 10,95,98,278 Persson P 10,95,98,278 Peters E 32,278 Pillai K C 10,95,278 Plaksin IN 2,7,12,279 Poling G W 47,70,124,270,273,278 Pollak RA 275 Pomianowski A 32,66,67,91,274,276 Porter R R 2,3,279 Pouchert C J 105,106 PovarennykA S 3,4,278 Pozzo R L 15-18,278,279 PrattAR 277 Prestidge C A 277,279,280 Price M D 274 Prince K E 278 Pritzker M D 32,42,92,270,279 Proctor C L 280 Prosser A P 12,272

Q Qin Wenqing 275,279,281,284,285 Qiu Guanzhou 274,275,279,281,283-285 Quinn M J 276

R Raghavan S 124,275 Ralston J 3,5,251,273,274,277,279

301

Index of Scholars Rand D A J 7,26,237 Rand D A 7,26,237,270,279 Ravitz S F 2,3,279 Rey M 14,201,279 Richardson P E 3,8,13,59,65,67,68,269-271,273-276,278-280,282 Rickard R S 88 Riemer S C 277 Riley K W 270 Roger C S 269 Rubio J 3,280 Russo S 277 Russo S P 269

s Sabacky B J 273 Salamy S G 2,280 Salvador P 12,280 SanchezV M 47,48,280 SandstromA 272 Sasaki H 227,280 SasakiTakehiko 280 Scaini M J 277 SchennachR 227,270 SchersonD 272 SchubertH 126,270 Schuhmann R Jr 65,273 Scott S D 276 Sebastian P J 278 SendaiA 280 Senior G D 282 ShafeevR Sh 278 Shafer M W 275 SharapovaN D 272 SheddKB 273 ShimoiizakaT 124,280 ShrammE 0 272 SkinnerW M 279,280 Smart R S C 273,278-280 Smith G W 49,275 Smith KA 49,271 SonmezI 5,284 SpeddenH R 273 StewartB V 273

302

Index of Scholars Stout J 280,282 Sun Shuiyu 275,280-282 Sun Wei 125,272,275-277,281,284 Suonien E

276

Sutherland K L

1,114,115,281

T Tafalla D 12,280 Taggart A F 1,281 Tai-Cheng Lee 271 Ta-Jen Huang 271 Takahashi K 242,281 Takeda M 9,88,277 ThielAG 279 Thornton E 15,201,281 Tolun R 7,9,57,70,92,281,282 Toperi D 70,92,281 Trahar W J 2,4-6,8,10,11,14,18,23,26,31,33,34,35,41,45-49, 56,57,59,60,65,67,68,73,88,95, 244,245,248,274,281,282 Trevino T P 269 Tryk D 272,274

u UsulAH

9,282

v Valli M 278 Vanghan D J 4,21,29,30,32,70,230,282 Vathsala K A 18,282 Vaughan D J 275 Verity B 277

w Wadsworth M E 2,271 Walker G W 59,65,68,277,278,280,282 WangDianzuo (WangD) 272,274,275,280,281,282 WangHui 274 Wang Jianqi 111,282 Wang S S 272,277 WangX 2821 WangXH 283 Wark I W 1,2,114,115,122,123,281,283 Watling K 283 Wei Q 275,283

303

Index of Scholars WieJM

271

Witika L K 283 Woodburn S I 275 WoodsR 2-5,8-11,23,26,41,42,45,46,68,70,88,92,150,237,269-271,273-276,278,279,283,284 WrightK 282

x Xiao Qi 279,283 Xie Y 17,282 Xiong Daoling 125,276,284 Xu Jing 275,279,281,284,285 Xu Shi 272 XuB 284

y Yarar B 73,284 Yarovsky I 277 Yeager E 272,274 Yekeler Meftuni 5,284 Yelloji Rao M K 16,251,284 Yin Q 275,276,282 Yoon R H 270,275-277,279,280,283-285 Young C A 150,283,284 YoungVY 150,278 Yu Runlan 279,284,285

z Zhan Xuehui 284 Zhang Qin 275,284,285 Zhang Shunli 275 ZhangD H 285 Zhao Jing 285 Zhuo Chen 285

304

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